[Feature request] Add a configurable -k option for any kind of branches.

[jillesmc]

I think all people who work with GitHub and use pull requests need this.

Recently Git Flow doesn't push after merging by default, causing pull requests to close. You can see this issue here:
https://github.com/petervanderdoes/gitflow-avh/issues/201

The workaround is to you use -k option when you finish a branch.

Please, insert this configurable option for any kind of branches.
Thank you ;)