There are various cases in which inline text fails to be separated by
(sufficiently many) newlines from adjacent block content.  A paragraph
needs a blank line (two newlines) separating it from prior text, as
does an underlined header; an ATX header needs a single newline
separating it from prior text.  A list needs at least one newline
separating it from prior text, but in general two newlines (for an
ordered list starting other than at 1, which will only be recognized
given a blank line before).

To avoid accumulation of more newlines than necessary, take care when
concatenating the results of converting consecutive tags to remove
redundant newlines (keeping the greater of the number ending the prior
text and the number starting the subsequent text).

This is thus an alternative to #108 that tries to avoid the excess
newline accumulation that was a concern there, as well as fixing more
cases than just paragraphs, and updating tests.

Fixes #92

Fixes #98

This is a partial alternative to #122 (open since April) for more
selective escaping of some special characters.

Here, we fix the test function naming (as noted in that PR) so the
tests are actually run (and fix some incorrect test assertions so they
pass).  We also make escaping of `-#.)` (the most common cases of
unnecessary escaping in my use case) more selective, while still being
conservatively safe in escaping all cases of those characters that
might have Markdown significance (including in the presence of
wrapping, unlike in #122).  (Being conservatively safe doesn't include
the cases where `.` or `)` start a fragment, where the existing code
already was not conservatively safe.)

There are certainly more cases where the code could also be made more
selective while remaining conservatively safe (including in the
presence of wrapping), so this is not a complete replacement for #122,
but by fixing some of the most common cases in a safe way, and getting
the tests actually running, I hope this allows progress to be made
where the previous attempt appears to have stalled, while still
allowing further incremental progress with appropriately safe logic
for other characters where useful.

This fixes various issues relating to how input whitespace is handled
and how wrapping handles whitespace resulting from hard line breaks.

This PR uses a branch based on that for #120 to avoid conflicts with
the fixes and associated test changes there.  My suggestion is thus
first to merge #120 (which fixes two open issues), then to merge the
remaining changes from this PR.

Wrapping paragraphs has the effect of losing all newlines including
those from `<br>` tags, contrary to HTML semantics (wrapping should be
a matter of pretty-printing the output; input whitespace from the HTML
input should be normalized, but `<br>` should remain as a hard line
break).  To fix this, we need to wrap the portions of a paragraph
between hard line breaks separately.  For this to work, ensure that
when wrapping, all input whitespace is normalized at an early stage,
including turning newlines into spaces.  (Only ASCII whitespace is
handled this way; `\s` is not used as it's not clear Unicode
whitespace should get such normalization.)

When not wrapping, there is still too much input whitespace
preservation.  If the input contains a blank line, that ends up as a
paragraph break in the output, or breaks the header formatting when
appearing in a header tag, though in terms of HTML semantics such a
blank line is no different from a space.  In the case of an ATX
header, even a single newline appearing in the output breaks the
Markdown.  Thus, when not wrapping, arrange for input whitespace
containing at least one `\r` or `\n` to be normalized to a single
newline, and in the ATX header case, normalize to a space.

Fixes #130
(probably, not sure exactly what the HTML input there is)

Fixes #88
(a related case, anyway; the actual input in #88 has already been fixed)

This improves the markdownify logic for cleaning up input whitespace
that has no semantic significance in HTML.

This PR uses a branch based on that for #150 (which in turn is based
on that for #120) to avoid conflicts with those fixes.  The suggested
order of merging is just first to merge #120, then the rest of #150,
then the rest of this PR.

Whitespace in HTML input isn't generally significant before or after
block-level elements, or at the start of end of such an element other
than `<pre>`.  There is some limited logic in markdownify for removing
it, (a) for whitespace-only nodes in conjunction with a limited list
of elements (and with questionable logic that ony removes whitespace
adjacent to such an element when also inside such an element) and (b)
only for trailing whitespace, in certain places in relation to lists.

Replace both those places with more thorough logic using a common list
of block-level elements (which could be expanded more).

In general, this reduces the number of unnecessary blank lines in
output from markdownify (sometimes lines with just a newline,
sometimes lines containing a space as well as that newline).  There
are open issues about cases where propagating such input whitespace to
the output actually results in badly formed Markdown output (wrongly
indented output), but #120 (which this builds on) fixes those issues,
sometimes leaving unnecessary lines with just a space on them in the
output, which are dealt with fully by the present PR.

There are a few testcases that are affected because they were relying
on such whitespace for good output from bad HTML input that used `<p>`
or `<blockquote>` inside header tags.  To keep reasonable output in
those cases of bad input now input whitespace adjacent to those two
tags is ignored, make the `<p>` and `<blockquote>` output explicitly
include leading and trailing spaces if `convert_as_inline`; such
explicit spaces seem the best that can be done for such bad input.
Given those fixes, all the remaining changes needed to the
expectations of existing tests seem like improvements (removing
useless spaces or newlines from the output).

This fixes problems with the markdownify logic for indentation inside
list items.

This PR uses a branch building on that for #120, #150 and #151, so
those three PRs should be merged first before merging this one.

There is limited logic in markdownify for handling indentation in the
case of nested lists.  There are two major problems with this logic:

* As it's in `convert_list`, causing a list to be indented when inside
  another list, it does not add indentation for any other elements
  such as paragraphs that may be found inside list items (or `<pre>`,
  `<blockquote>`, etc.), so such elements are wrongly not indented and
  terminate the list in the output.

* It uses fixed indentation of one tab.  Following CommonMark, a tab
  in Markdown is considered equivalent to four spaces, which is not
  sufficient indentation in ordered list items with a number of three
  or more digits.

Fix both of these issues by making `convert_li` handle indentation for
the contents of `<li>`, based on the length of the list item marker,
rather than doing it in `convert_list` at all.