Added ability to call Eloquent::with on a Relationship instance

This PR allows for calling `Eloquent::with` even after a relationship query has already started.

I added this for myself a couple months ago, I'm not quite sure if this can be moved up to the `Eloquent\Query` class. That however would not have much benefits I guess, as this extension is mainly useful when you want to eager load a relationship, which isn't currently possible.