Configurable Markdown extensions

This lets the YAML configuration specify a list of Python Markdown extensions
to enable (on top of the default set). This is especially handy, for example,
when you want nice typography via the SmartyPants extension.
Let's leave it as-is.  Just wrapping this in a set() wouldn't guard against multiple invocations [if one of them had parameters](http://pythonhosted.org/Markdown/reference.html#extensions), and sets sort their contents:

    >>> range(5,1,-1)
    [5, 4, 3, 2]
    >>> set(range(5,1,-1))
    set([2, 3, 4, 5])

Of course, mine isn't the opinion that counts.  I really hope this and #97 get merged whenever @tomchristie next has a chance to hack on this. :)

If I have a chance later this week, I may send a pull request against your branch to flesh out the docs re: passing parameters to Markdown extensions.