Add config for custom repository factory

The idea is to be able to use a custom repository factory (implementing Doctrine\ORM\Repository\RepositoryFactory) so, for example, custom repositories can be resolved out of the IoC container.
The repository factory itself is resolved out of the IoC container.

I'm using this right now to have a custom repository factory that resolves custom repositories out of the IoC container using composition instead of inheritance (my repositories don't extend from the docrine's EntityRepository, but contains it).

I hope I'm not forgetting anything... again :smile: At least is working for me so far.