[RFE] Add support for pushing bugs to remote tracker

I'm trying to use taskwarrior + bugwarrior to simplify bug-management over multiple bugtrackers, and from what I searched, it seems bugwarrior doesn't support pushing bugs to remote trackers, only `pull`ing.

Surprisingly though, I haven't found an RFE for this feature, neither I found *(in case it is a conscious decision)* any mention in documentation that bugwarrior has no such support. So I figured it might be a good idea to create this RFE either way.

## Expected behavior

`bugwarrior push` command as a counterpart to `bugwarrior pull` — one of the possibilities, though there may be even better ideas.