Before this change, `git-flow feature start` would nag about the working
tree being dirty and disallow continuation. Then, there were two
alternatives:

- stash away your changes manually, create the new branch, restore the
  stash; or
- use `git-flow feature start -f` to force creation

There is absolutely no good reason for git-flow to forbid this creation,
as git allows it already.  Therefore, the -f behaviour is now the default,
without an option to turn that behaviour off.  Git takes care of unsafe
situations again now.