Git Kill

@charlierudolph this implements #87 
Ah, cool, yes that achieves the desired output more directly. In regards to asking for confirmation, I have come to the conclusion that given how easy it is to undo a branch delete, I'd rather provide the possibility to run `git kill --undo` in case I want to revert a kill. Given that, we need to keep the open changes that existed before running `git kill`. The only way to keep them that I can think of is to commit them into a temp commit. So I'm staying with my version for now, but solely to enable "undo" later.