Add this container :attr:`settings` to an existing ``parser``.

def add_to_parser(self, parser):
        """Add this container :attr:`settings` to an existing ``parser``.
        """
        setts = self.settings

        def sorter(x):
            return (setts[x].section, setts[x].order)

        for k in sorted(setts, key=sorter):
            setts[k].add_argument(parser)
        return parser