feat: git-bug configuration

[MichaelMure]

@adamslc said:

What are your plans for a git bug config command? To start off the conversation, here is an incomplete list of things that might possibly need to be configured. Note that I'm not arguing that everything on this list should actually be implemented.

* Default remote
* Set of possible labels
* Possibly move `git bug bridge configure` under here?
* Default new bug and comment templates
* Keybindings in `termui`
* Author data specific to git bug. I'm not sure if this exists?

Also, I think that having connivance flags to update the config from other commands makes sense. In #62 I suggested using -d and --set-default in git bug push to get and set the default remote. I'm now convinced that those aren't the right flags to use (looks to much like delete), but I still think that this is important functionality. Perhaps mirroring git push and using -u and --set-upstream would makes sense.