Add :cabal Ex command to Vim2

The :cabal command runs `cabal build`, which allows me to run `:cabal` and
then `:reload` from with yi in order to incorporate the latest changes I've
made. Or if `cabal build` fails to press enter on the error message line and
be taken to the source of the error.
:cabal supports passing arguments to cabalBuildE
Merge pull request #492 from benarmston/vim2-cabal-ex-commands

Add :cabal Ex command to Vim2