More Markdown Goodness :)

@pearkes you might like this. This PR adds more syntactic sugar to our Markdown parsing as well as fixes a few bugs I have seen pop up. 

The [README Rich Diff](https://github.com/hashicorp/middleman-hashicorp/pull/4/files?short_path=04c6e90#diff-04c6e90faac2675aa89e2176d2eec7d8) pretty well summarizes the changes.

/cc @mitchellh @joshfng @armon @ryanuber @KFishner too :)