SoC proposal: Implement Cabal configurations

[Björn Bringert]

I want Cabal configurations, so I added a SoC proposal for it, see 
http://hackage.haskell.org/trac/summer-of-code/ticket/1131

This was the latest spec I could find: 
http://www.mail-archive.com/cabal-devel@haskell.org/msg00282.html

I haven't taken part much in the configurations discussions, so I'm not 
sure how finished the spec is or if it would be a good SoC project. Feel 
free to comment on / change the proposal as you see fit.

/Björn