git flow init: Let me change the "Initial commit" message

[geekley]

When running git flow init, I'd like to have a way to set/edit the "Initial commit" message to something else before the commit, without having to amend the commit and reposition the master branch afterwards. Best solution would be to make git-flow-init ask for the initial commit message, allowing that [Initial commit] (or whatever else is configured) as default. That's a good solution because it also makes it clear to the user that an initial commit is going to be made.