Added "fenced_code" to markdown default plugins; possibility to add extra Markdown plugins using `config` file

This pull request has few changes but is very useful to me.

Enhancements:
* Added the `fenced_code` Markdown plugin (it was already in Markdown2 but not Markdown!)
* Using `codehilite` instead of `highlight` as Markdown CSS class (it's Pygments' default, why was it changed?)
* Users can add extra plugins using `markdown_extra_plugins` in the config file. Defaults to empty list. Renderers are monkeypatched to add the extra plugins when `generate_site` is called.