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)
>
> > +
> > +  defp split_into_columns(line) do
> > +    (if String.ends_with?(line, "|"), do: String.strip(line, "|"), else: line)
> > +    |> String.split(~r/\s\|\s/)
>
> We could break this using a string instead of regex.
>

I believe it has to split on any whitespace...