Store github usernames

See #8 for much backstory.

@mattbaker the knockdown argument for storing the github usernames in the git config is that if you use this command to set just _one_ user then the github username is _discarded_ which would prevent building tools on top of this that assume a github username can be retrieved from the workstation.

So this commit removes the DBC specific stuff but leaves the caching **in** because there's now a documentable reason for it.