Experimental front-matter support

The current implementation just rendered the front matter as a
code block.

Outstanding issues:

* Should I try to render the front matter into a table? Need to
  understand more about this; if it'll always be as simple as
  shown in the examples, I can just parse it; if we REALLY need
  to parse it as YAML then we might have a problem here.

* If we don't/can't render it into a table, we should at least
  syntax-highlight it if we're allowed. Maybe I should submit a
  YAML highlighter to Prism?

* The PEG syntax highlighter needs fixing. We should probably do
  it in the highlighter, not the PEG grammar.
Implement object to HTML table rendering
YAML => table rendering

Is there a way to make the keys ORDERED? Maybe we need to drop
back to LibYAML with the Objective-C wrapper. :(
Customize YAML.framework to use ordered dicts
Make setext title 2 require FOUR dashes
Add licenses

LibYAML (MIT)
M13OrderedDictionary (MIT)
YAML.framework (MIT)
Refactor front-matter generation method signature
Merge branch 'front-matter'

Fix #14

Conflicts:
	Podfile