Add NOT feature

[sottwell]

It would be nice to be able to list resources that do NOT have a given tag.

[davidpede]

+1

[Jako]

In MoreGallery, the excluding tags are handled with:

                $excludedTagIds = implode(',', $excludedTagIds);
                $c->where(array(
                    "NOT EXISTS (SELECT 1 FROM {$this->modx->getTableName('mgImageTag')} Tags WHERE `mgImage`.`id` = `Tags`.`image` AND `Tags`.`tag` IN ({$excludedTagIds}))"
                ));

Tagger could do it similar.