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: 
https://github.com/Originate/git-town/blob/kg-git-kill-branchname/features/git-kill/supplied_branch/feature_branch_with_open_changes.feature#L11

I think this is a legitimate use case, and we should think about using conflicting commits everywhere where there are open changes in specs. I also think that it is more robust if our spec suite switches branches cleanly. Switching branches cleanly means doing it without open changes. Many Git Town commands are based on the same principle, for the same reason (robustness). I don't want to have false negative specs just because they are too lazy to stash away changes before switching branches around.