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)
I made it so that two successive lines must look like table lines.

I really, really don't like the heading requirement.  I often have the need for tabular material with no headings. For example, I feel that options should probably be in tables, not lists, but only if there's no heading.