Epic: Support JIRA

[JKrag]

MVP: A seperate tool with same semantics, i.e. you would probably only have either the JIRA version or the GitHub version installed.

This could probably be implemented in the current alias-based version, but should probably wait until we have a real extension.

If we are still in the alias based model, then then aliases could obviously be per repo, and thuis you could have both on the same machine, but would have to add aliases yourself for each new repo.