Add patch support

[vmrob]

It would be desirable to be able to apply a list of patches to a source directory prior to project type determination and prior to a build. Currently, the best way to do this varies between download types (git apply {{ needs_file|dirname }}/some-patch.patch, patch -p1 < some-patch.patch, among others) and a unified syntax for applying this without the boilerplate would be greatly desired.

Patch file contents should be factored into the configuration hashes.

[ccbrown]

Related: #110