Black-box testing

[smashwilson]

Create a framework for black-box testing of the Jekyll preparer behavior. I'd like to have a bunch of directories containing Jekyll repositories, paired with directories containing the expected results of preparing them. Running the tests (locally or through Travis) will invoke the preparer on each source directory, compare the actual results with the expected ones, and report any differences found for each or any errors raised during the preparation.

• In non-live mode: emit prepared documents to disk in a flat directory of files named with the URL-escaped content ID of each document.
• In non-live mode: copy assets to an assets/ subdirectory of the site destination.
• Create a Jekyll site in a subdirectory that exercises the most basic functionality: render some markdown, a document with full frontmatter, a document with minimal frontmatter, an asset to recognize.
• Create the expected output.
• Author a test script that'll prepare and compare the two.
• Activate Travis.

Part of deconst/deconst-docs#55.