Look for a .zuurlc in process.cwd() and ancestors before $HOME

This implements a local lookup for a `.zuulrc` as mentioned in #82.

I couldn't see any tests for the bin script so I haven't added tests. Let me know if that's an issue or not.

I wrote a module to do the lookup that has full test coverage: https://github.com/bengourley/find-nearest-file/blob/master/test/find-nearest-file.test.js (the existing ones I found didn't have cross platform support and weren't well tested).

This doesn't do any extending of options found in different places, it simply uses the first `.zuulrc` that it finds.

Hope this is ok!