Adds linum-mode to prelude-prog-mode-defaults.

I think it's very helpful to have line numbers by default when working with code. Most text editors feature it and people expect them to be there. I know that many people use Emacs for much more than text editing it so I added this feature only to prelude coding hook. If this feature goes against the Prelude vision I understand and it's ok. This is just a simple change.