Add arguments for this command.

def add_argparser(self, root, parents):
        """
        Add arguments for this command.
        """
        parser = root.add_parser('diff', parents=parents)
        parser.set_defaults(func=self)

        parser.add_argument(
            '--secrets',
            dest='secrets', action='store',
            help='Path to the authorization secrets file (client_secrets.json).'
        )

        parser.add_argument(
            '-d', '--data',
            dest='data_path', action='store', default=None,
            help='Path to a existing JSON diff file.'
        )

        parser.add_argument(
           'report_a_path',
            action='store',
            help='Path to a JSON file containing the initial report data.'
        )

        parser.add_argument(
           'report_b_path',
            action='store',
            help='Path to a JSON file containing the report data to compare.'
        )

        parser.add_argument(
            'output_path',
            action='store',
            help='Path to output either an HTML report or a JSON data file.'
        )

        return parser