git kill branchname

@charlierudolph Aaaaahhhh, for some reason some commits meant for this PR ended up in an already merged PR. I was messing around with `git extract` while being distracted on the resource planning call, so that's my (super lame) excuse... Anyways, please take a look at https://github.com/Originate/git-town/commit/738e3e76158a4803c5695abf71349b21d02033f5, and comment there. I will implement the corresponding fixes in this pull request, and once this gets merged its all going to be kind of okay again...  :smile: 
I would think we would call ensure_has_branch before ensure_is_feature_branch.
This currently works because a non-existing branch is still technically a feature branch (we just check its not the main branch or a non-feature branch).

Maybe put ensure_has_branch inside of determine_branch_to_kill?