(cherry picked from commit ca5f604)

In order to make it easier to upgrade to the new syntax, we'll release fluent
0.4.3 which will keep the API compatibility with 0.4.2 and will be able to read
Fluent Syntax 0.5 files (just like fluent 0.6.0).

These changes make fluent 0.4.x able to read both Syntax 0.4 and Syntax 0.5 files:

  - Supported Syntax 0.4 features:
    - `//` comments.
    - Sections (`[[ Foo ]]`).
    - Message definitions may omit the `=` after the identifier if they don't
      have a value.

  - Supported Syntax 0.5 features:
    - `#`, `##` and `###` comments.
    - Identifiers starting with a dash (`-`) are parsed as terms.

The only feature of the old Syntax 0.4 which isn't supported anymore are Tags.
In practice we didn't see any of the current consumers of `fluent` 0.4.2 take
advantage of tags. In Syntax 0.5, [tags were replaced by attribute defined on
terms][terms].

[terms]: https://github.com/projectfluent/fluent/blob/master/spec/CHANGELOG.md#050-january-31-2018

Backport MessageContext parser changes from fluent 0.6.0

  - Correctly parse empty comment lines. (#149)
  - Forbid null attribute and variant values. (part of #150)

Backport MessageContext parser from fluent 0.6.2