Register a converter

        :param string entry_point: converter to register (entry point syntax)
        :raise: ValueError if already registered

def register(self, entry_point):
        """Register a converter

        :param string entry_point: converter to register (entry point syntax)
        :raise: ValueError if already registered

        """
        if entry_point in self.registered_converters:
            raise ValueError('Already registered')
        self.registered_converters.insert(0, entry_point)