Support literate haskell

Just copy pasted from here

http://www.haskell.org/haskellwiki/Literate_programming/Vim

![selection_007](https://f.cloud.github.com/assets/294349/1090337/244ec45a-1655-11e3-8035-b3b1d3461821.png)

Note how the `begin{code}` stuff works now.