Markdown renderer

Here at Draftable, we really like Markdown -- some would even say we like Markdown more than we like reStructuredText! ;)
So we took the liberty of adding in a MD renderer to go with the rST one; it assumes all previous articles have been written as rST but gives users the choice of adding new ones in MD format.

Currently there are no test cases for this (aside from the ones I ran locally just playing around with my own browser), but let us know if you'd like these. Otherwise, if you feel like this is missing something let us know and we'll add it in.