Allow extend fillable and rules

[ezandonai]

Today already exist a method to get rules, so it's possible extend models and reuse the rules with new rules merging inside the method.
But the fillable property hasn't a method to get the value, so this way we can't extend this property on extended model.

Would be good if was possible to extend models and reuse the fillable and rule properties, this way will be better used the polymorphic feature.