Add support for multimarkdown or github flavored markdown

[durden]

Currently we support vanilla markdown syntax, which is great. However, markdown really falls short when you want to do something like provide footnotes or citations. Luckily,
multi markdown doesn't have this problem. It builds on the standard markdown format.

The downside of this is that I don't think multi markdown syntax is standardized like the basic markdown. So, a lot of places have their own stuff built on top of it, etc. We might need to be more explicit about supporting multi markdown in our UI if we did this.

We could also try supporting github flavored markdown which would git ride of this issue as well. I don't think this supports footnotes and citations though.

We should look into how github does this and go with their style/code/etc. since we are already so tightly integrated with them.

For reference, jeff atwood has tried complaining about markdown standardization, etc. before and haven't seen much traction.