GetBuilder() method for models manager

[TheAlg]

I think it would be interesting to have more flexibility using builder, like adding new queries to the current builder instead of creating new one eache time. for example, I would like to add new queries after verifiying parameters passed to url, something like
if ($param !== null){
$this->model->manager->getBuilder()->join(...)
}

[niden]

@TheAlg Can you open an issue for this please?