Support for custom configurations

[kenny1847]

This relates to #196.
There is going thread about persistent global config, but, on the other hand, as for me, in many cases I need custom .rgrc for custom project. And it will be great pros, because anyone invited to project won't has to write own aliases or anything what he/she likes.

[BurntSushi]

I don't think we need two tickets tracking this. If directory specific configs are desirable, then that should probably be part of the initial design, since I expect it will add an extra layer of complexity.