I had a version with just re-exporting command in ``mopidy.backends.base``, but commands are tied to extensions not backends so was a bit off. Also the way it is now we need to be able sub-class the command with a ``run(self, args, config, extensions)``.

So alternative could be to remove the ``run()`` from the Command class, pass in an instance and either do ``return cmd, callable`` or some other way of hooking it up. Of course it messes things up with respect to multiple sub-commands mapping to on callable, so perhaps ``cmd.set_target(callable)``?