Is it possible to move QueryBuilder logic to trait? To support custom Builders

[ievgenklymenko]

Hi!

The practic of extending builder through newEloquentBuilder is becoming more and more common.
Package owners and project devs - everybody wants to use their own builders.

But since this package uses full custom QueryBuilder class, it is hard to combine with other custom builders.
If we had all builder logic inside a trait instead, this would save the day.

It sound like not hard to do - neither does it ruin backward compatibility - unless I'm missing something.
There was this old request showing how it can be done (but it is stale now):
#229