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)
>
>  safer way to know if something is a table or not is by checking the
> *next* line and see if it is made only of constructs like ----- | -----
> and so on.
>

I'll change it to look ahead. However, I don't want to insist that every
table has a heading.