Add "cell for view model" API to binding section controller

We should have a public API to get a cell given a view model in the [binding section controller](https://github.com/Instagram/IGListKit/blob/master/Source/IGListBindingSectionController.h).

Question, how do we make this an O(1) operation? Right now we store an [array of view models](https://github.com/Instagram/IGListKit/blob/master/Source/IGListBindingSectionController.h#L72), but we'd have to do `indexOfObject:` to get the index, which is O(n).