Add HTML::Pipeline to core

[HTML::Pipeline](http://github.com/jch/html-pipeline) is the :bee:'s knees. One day @benbalter was like

> what are your thoughts on submitting html-pipeline to Jekyll core as a markdown renderer

and I was all

> i am very much :+1: for it

So here we go.

Much to my surprise this actually works pretty well. I took [a cue from how nanoc accepts filters](https://github.com/burnto/nanoc-html-pipeline/pull/1), wherein you can basically pass in an array of filter names, and The System will figure out what you mean.

I can't figure out how to properly test for failures (they're written in the test), so help on that would be much appreciated.

Todo:

- [x] Write test for mentions
- [ ] Actually test for failing items
- [ ] ~~Support custom filters~~ Will probably need to be handled by a plugin, not core.
- [ ] Update docs to list html_pipeline defaults

/cc @parkr