Add \Illuminate\Eloquent\Collection::modelValues() to get an array of model attributes

It would be nice to be able to pluck attributes from collections in the way you can pluck primary keys with Collection::modelKeys

I wasn't sure about the overly complex set of method signatures I implemented, so it might be worth deciding on one approach or another (if you're even interested in including this, of course!)

I guess my preference would be that you give a string to get an array of strings, and you give an array to get an array of arrays.  That would mean dropping the method signature that lets you pass multiple parameters (currently it behaves the same as passing a single array).