Add a run-mode where chat takes place under the hood in an existing repo

[JKrag]

For other use cases than my original "send links to myself", it might be relevant to have chats run "under the hood" in an existing repo with e.g. real project code in it. This could probably easily be achieved by pushing chat "channels" to refs/chat/ instead of refs/heads/, and thus being transparent to normal users of the repo.

This seems reasonably doable, but I don't quite know yet how big the impact would be on the current code/logic, so I might decide against this feature if it ruins the simplicity of the current implementation.

[JKrag]

Could also just be in normal branches called chat/* and with a start option for GitSlick --namespace "chat"