Support for --config and --docker-<options>

It's a new feature branch on top of https://github.com/shawnsi/docker-rpmbuild/pull/4 . 

So handle that one first, then this diff should hopefully look a bit nicer and not include the diffs from that pull request. 

Unsure if I plugged the setuputils correctly for the tests, but I guess you could always fix and tweak :-)