Change liquid converter order

Second attempt ([see first](https://github.com/mojombo/jekyll/pull/895)). Now features less-terrible commits, more unit tests, and `Converters::Include`'s conversion has been separated into its own method.

* Liquid tags are now parsed **after** converters are run on files.
* Liquid tags are encoded as HTML comments (sample:` <!--LIQUID-a4ef62-2-->`) during conversion, using the new `LiquidEncoder` class.
* `{% include %}` liquid tag will now run included files through converters if any match their extension
* Docs have been updated to explicitly mention order of parsing for files with YAML front-matter, and to mention that included files are converted if their extension matches.