(Unstable) Shortcut for plugins to define own config parameters.

def _define(self, key, default=_UNSET, help=_UNSET, validate=_UNSET):
        """ (Unstable) Shortcut for plugins to define own config parameters. """
        if default is not _UNSET:
            self.setdefault(key, default)
        if help is not _UNSET:
            self.meta_set(key, 'help', help)
        if validate is not _UNSET:
            self.meta_set(key, 'validate', validate)