Add pager support to help output.

def print_help(self, *args, **kwargs):
        """ Add pager support to help output. """
        if self._command is not None and self._command.session.allow_pager:
            desc = 'Help\: %s' % '-'.join(self.prog.split())
            pager_kwargs = self._command.get_pager_spec()
            with paging.pager_redirect(desc, **pager_kwargs):
                return super().print_help(*args, **kwargs)
        else:
                return super().print_help(*args, **kwargs)