Git Flow: add flag to set --no-ff when closing feature / release / hotfix

Git Flow: add flag to set --no-ff when closing feature / release / hotfix.

I think that adding a such checkbox on a form "Finish *" will be enough.

It'll be really useful for every developer that uses such branching model: http://nvie.com/posts/a-successful-git-branching-model/
with SourceTree.