feature request: remove old dura branches periodically

[dswij]

I think it's good to have a subcommand to remove old dura branches to clean up the clutter in git logs. Preferably, I can select the set of dura branches to be removed by time passed.

I'm not sure about removing it periodically, but at the minimum it's good to have a way to automatically remove dura branches.

[tkellogg]

Most discussion on this is happening on #9. Another idea is #31. This is probably one of the biggest problems faced by dura at the moment.