Set a string option.

            Args:
                option (str): name of option.
                value (str): value of the option.

            Raises:
                TypeError: Value must be a string.

def set_string(self, option, value):
        """Set a string option.

            Args:
                option (str): name of option.
                value (str): value of the option.

            Raises:
                TypeError: Value must be a string.
        """
        if not isinstance(value, str):
            raise TypeError("%s must be a string" % option)

        self.options[option] = value