Add basic support for markdown tables to built-in formatter

It turns out that for the simple cases we _can_ interpret the markdown inside table cells.

The output looks like this:

![1____play_dave-elixir__beam_smp_](https://cloud.githubusercontent.com/assets/10648/3621338/9ae27560-0e13-11e4-9492-2897c4f0a1d2.png)

(The next PR will rationalize the spacing between block level elements)
Well, if the convention is good or not is another discussion that we could tackle separately. The whole codebase should follow a single convention though, if every pull request adds its own style, it gets harder to review, the codebase becomes messy, etc.