Add help command as per ticket #272
Add help command as per ticket #272

"cabal help" behaves like "cabal --help"
"cabal help cmd" behaves like "cabal cmd --help"
Should still work with command line completion.

20080430133740-adfee-f518b22528deb232f63730da91923f44f3951274.gz

dcreager/cabal,dcreager/cabal