Allow setting default repository per room/channel using hedwig_brain

[brienw]

This would allow each channel to have a memory of the current repository. Once a repository is set, all future commands need not specify a repository. github prs labzero/hedwig_github would abbreviated to github prs, for example.

Proposed commands:

1. github repo labzero/hedwig_github: set the repo default for this channel to labzero/hedwig_github
2. github repo: print the current value for default repo for this channel
3. github repo forget: forget the current default repo