Return options that can be used given
        the current cmd line

        rtype: command.Option generator

def available_options(self):
        """Return options that can be used given
        the current cmd line

        rtype: command.Option generator
        """
        for option in list(self.cmd.options.values()):
            if (option.is_multiple or
                    option not in list(self.used_options)):
                yield option