Support more Markdown syntax

[dbanty]

Using basic string matching has been okay for now, but there are too many ways to do things in Markdown for us to keep that up. We're going to have to go back to parsing Markdown properly, unfortunately, but hopefully with a streaming parser of some sort (if it exists). Specifically, you're allowed to have titles that look like this in Markdown:

First Header
=========

Second Header
---------------

This is just not supported by Knope right now. Eventually, if we want broader adoption, we need to adapt even more to whatever format folks want.