Protect against incorrect init function call.

Only call the init function if the function is defined, instead of maintaining a list of commands that define it.  This fixes a bug with the "git flow version" command.

There are already multiple pull requests to fix this bug, for example https://github.com/nvie/gitflow/pull/262 and https://github.com/nvie/gitflow/pull/237), but this one avoids hardcoding the list of command names.