RFC: Adding git prefs to 'cabal init' author-lookup heuristics

This patch extends 'cabal init' to make guesses about the author from *git* preferences, as well as darcs preferences.

I'm submitting this patch for feedback and guidance. Some notes:

* The original heuristics take precedence.
* I use `System.Process.readProcessWithExitCode`. I leave it to more experienced hands to decide whether this is sound or not; I have no idea.
* I added no tests. Yet. `cabal-dev configure --enable-tests` fails and I am unsure how to proceed.
* I only added haddocks to the 'bigger' functions. I'm not even sure the remainders should be top-level definitions. Again, I figure this is a stylistic choice and I'm better off asking what the preferred standard is. :)