Allow repo specific configuration

[PaulFarver]

It would be great to allow repository specific settings.

A possible solution to this could be to include a configuration file in the root of the repository. Something along the lines of a .bulldozer file for https://github.com/palantir/bulldozer

Configuration keys:

• Target branch implemented in Support target branch configuration #20
• Major label implemented in Configuration expansion #21
• Minor label implemented in Configuration expansion #21
• Changelog regex for pull requests
• Changelog template for releases
• Feature toggling (Promotion, cleanup, pre-releasing)
• Full release implemented in Configuration expansion #21
• Autolabeling