This provides a mechanism for adjusting the behaviour of these
relations when building more complex data models.
For example the following example has a status field incorporates a
Status field into the relationship:
```php
function MyRelation() {
$rel = $this->getManyManyComponents(‘MyRelation’);
$rel = $rel->filter(‘Status’, ‘Active’);
$rel->addCallbacks()->add(function ($relation, $item, $extra) {
$item->Status = ‘Active’;
$item->write();
});
}
```
Introduces a new library dependency: http://github.com/sminnee/callbacklist
