Add custom args to the email

        :param value: A list of CustomArg objects or a dict of custom arg
                      key/values
        :type value: CustomArg, list(CustomArg), dict

def custom_arg(self, custom_arg):
        """Add custom args to the email

        :param value: A list of CustomArg objects or a dict of custom arg
                      key/values
        :type value: CustomArg, list(CustomArg), dict
        """
        if isinstance(custom_arg, list):
            for c in custom_arg:
                self.add_custom_arg(c)
        else:
            self.add_custom_arg(custom_arg)