Force deletion of local branch, ignoring unmerged changes when cancellin...

...g a feature

When cancelling a feature, the local branch isn't deleted due to unmerged changes, e.g.:

    $ git hf feature cancel test_feature

```
To git@github.com:user/repo.git
 - [deleted]         feature/test_feature
error: The branch 'feature/test_feature' is not fully merged.
If you are sure you want to delete it, run 'git branch -D feature/test_feature'.
```