Optional markdown output.

I don't know if this is an edge case or genuinely useful, but I have the following use case:

 1.  I write my JavaScript documentation with docco.
 2.  I host the resulting documentation on GitHub Pages.
 3.  I try to write my documentation so it can also serve as a README.

As a result, I wanted to automatically produce markdown and html from my source code.  Docco already parses out the markdown as an intermediary step, it's just not writing out the resulting markdown.  This pull request adds optional markdown output using only a handful of lines.

    $ docco -m src/example.coffee

    # Slightly shortened for readability.
    $ docco: example.coffee -> example.md
    $ docco: example.coffee -> example.html

There are likely other use cases involving Jekyll or GitHub Pages that are just not crystal clear to me at the moment.