Add the necessary plumbing for registering cabal files and giving them a proper icon.

I don't have an answer to how much support we need for cabal in the long run, but we should at least recognise that it's widely used and show that we therefore care. 

This patch adds the necessary plumbing for IntelliJ to recognise cabal files and give them their own icon. I added a cabal subdirectory to prevent over-crowding with the Haskell plumbing. I also added a (real) cabal file in the test directory for future reference. It's probably of medium complexity compared to other cabal files: it uses a bunch of the available options and fields and additionally features non-ascii characters in the copyright fields.