Change liquid converter order

[jyruzicka]

Second attempt (see first). 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.

[jyruzicka]

Upon installing this on my local machine, maruku started spitting errors involving having raw html on a line with plaintext. Am also experiencing some inconsistencies with {% highlight %} liquid tags. Teach me to think the code is clean based purely on unit tests. I'll see if I can work out some fixes for these issues.

[parkr]

We'll have to wait until all the tests pass to consider this.

[jyruzicka]

No worry. I've actually had to back off and approach this from a different angle anyway. Unit tests + travis passing locally, I'll chuck some info into the docs on how to do conversion before liquid tags are parsed, then issue a new (third) pull request.

This time.