Add support for patches

[Dustin Frisch]

niv should allow to apply local patches for sources. This would avoid to maintain a tree for sources that exist only to track patches.

I can think of multiple ways to implement this:

• By providing a wrapper for each source derivation that allow to apply patches (something like sources.nixpkgs.patched [ ./path/to.patch ]
• By adding a config option listing the patches to be applied

I would prefer the first option and would be happy to work on an implementation if this is an acceptable approach.