Change liquid converter order

* 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 `include`d 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 `include`d files are converted if their extension matches.