For some reason, :code seems to add an empty line at the end of its
code block. This uses Markdown code blocks instead, although in order
to work around a RedCarpet bug it has to indent them strangely.
* Add htmlproofer to verify links
Partially addresses #247
* Make htmlproofer a Rake task
* Verify that we don't have leftover Markdown links
* Don't build the full Ruby Sass docs before running htmlproofer
* Don't check generated documentation
* Fix a few more broken links
* Add another URL exception
* Limit which branches we run tests for
* Add more URL exceptions
