Add preference for using tor only when having power

(Sorry if this is a dup; I searched but am not confident.)

I just built and installed Briar after being absent for a while. I see there's a preference for using tor never, on wifi, and on cellular. That's great for some, but doesn't address my problem. I realize there is tension with UX and complicated config, but given that having a HS is very very costly power wise, and outbound tor is somewhat costly, I'd like to see a config for "Use Tor when not charging" that is "no, outbound only, outbound and inbound". This would probably let me start running briar, and I suspect would let others do so as well.