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/)
> > +    |> Enum.map(&String.strip/1)
> > +  end
> > +
> > +  defp pad_to_number_of_columns(cols, col_count)
>
> This check is not necessary as the clause below would return the proper
> result regardless
>

I'm not sure I understand:

iex(1)> String.split("|A|B|", "|")
["", "A", "B", ""]
iex(2)> String.split("A|B", "|")
["A", "B"]
iex(3)>