Add whereMorphs to query builder

[monove]

When we want to query a table that morphs to other tables we need to write code like

->where(function($q) use($instance) {
  $q->where('model_type', get_class($instance))
    ->where('model_id', $instance->id);
}

It would be nicer if we could simply do

->whereMorphs('model', $instance)