Add a ``mkdocs json`` that builds a site in json.

This is mainly for Read the Docs -- but it would be neat to have mkdocs build both the JSON and HTML versions of docs, so when you hit a URL, you can just change the .html into a .json and you get the JSON representation of the files.

This doesn't have tests currently, just curious what folks think about it before I invest too much time.
'json' needs to be added to the help output in the else clause a few lines down.