Allow new feature branches in dirty working trees.

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.

git-flow-feature

@@ -172,15 +172,11 @@ parse_args() {
 
 cmd_start() {
 	DEFINE_boolean fetch false 'fetch from origin before performing local operation' F
-	DEFINE_boolean force false 'force creation of feature branch (ignores dirty working tree)' f
 	parse_args "$@"
 	BASE=${2:-$DEVELOP_BRANCH}
 	require_name_arg
 
 	# sanity checks
-	if noflag force; then
-		require_clean_working_tree
-	fi
 	require_branch_absent "$BRANCH"
 
 	# update the local repo with remote changes, if asked