Make Dotbot usable as a Python library

[anishathalye]

If dotbot is easily usable as a Python module (not just through the CLI), people could design more complex setups directly in Python. That means you get the full power of Python and the ease of use of dotbot, without adding support for more complex configuration files.

(via https://github.com/anishathalye/dotbot/issues/35#issuecomment-185997165)

CC @bchretien @MaikuMori