Set default args for commands in collection.

        Default args are used when the corresponding args aren't passed
        on the command line or in a direct call.

def set_default_args(self, default_args):
        """Set default args for commands in collection.

        Default args are used when the corresponding args aren't passed
        on the command line or in a direct call.

        """
        for name, args in default_args.items():
            command = self[name]
            command.default_args = default_args.get(command.name) or {}