Adds rabl support.

Source code is not by me. It comes from [yaymukund][https://github.com/yaymukund/vim-rabl].

So what is [rabl][https://github.com/nesquena/rabl]? It's an ruby API builder for JSON and XML files. So all *.rabl files are basically ruby files so this code will actually set rabls filetype to ruby and add some improved syntax highlighting.