Github Flavoured Markdown

It would be great to be able to use [GitHub Flavoured Markdown](https://help.github.com/categories/writing-on-github/).

Looking at the [Pandoc options](http://pandoc.org/MANUAL.html#general-options) you can use `markdown_github`, but trying to use this in my config file gives me an error:

```
gitit: Unknown page type: markdown_github
```

Searching the code, it seems this happens in [Utils.hs](https://github.com/jgm/gitit/blob/master/src/Network/Gitit/Util.hs#L107), but I don't know enough (read: any) Haskell to figure out where to go from here, and how to go about adding support.