Accept top-level ".bobconfig" for repo globals

[rafecolton]

Since the user has limited control over the data sent with a Travis/GitHub webhook, certain options will have to be specified statically in the repo.

The suggested implementation for this is a .bobconfig file at the top level of the repo. Example options to be included in the .bobconfig are the default Bobfile and container group to be built (ala #96)

[jszwedko]

What line would we draw between what should be in .bobconfig vs what goes in the Bobfile?

[rafecolton]

So for sure the things that are specific to build-time would go in there. For example, we can't set a default Bobfile in the Bobfile itself. Setting the container group in there also makes sense, as that refers to which container(s) we build, not what groups are applied to the [[container]] sections.

After that, I'm not sure. There are some things that would seem handy to put in there like the docker registry credentials. However, that doesn't really fit the "specific to build-time" criteria, and I would like to avoid added complexity that would come from having too many levels of precedence. We already have a lot with [[container]] > [container_globals] > command-line options > environment vars

What are your thoughts on it?