Allow for Custom Collector methods

[xy2z]

Eg. a magic method __call() - try to call the custom class/method with $this->data as argument.

Or set the CustomCollector class via the core config, so it points to eg. CustomCollector::class, which should be extended of Capro's Collector class.

It should still return $this, well maybe not necessarily, as it could also be like a sum() method, so probably let it be mixed.