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
>
I agree. But I also worry that "this isn't how we do things around here" is
not a great way to stimulate debate on possible better ways.... :)

Changed the source.