Add prefetching functionality to IGListBindingSectionController as WorkRange for IGListAdapter

## New issue checklist

- [x] I have reviewed the [`README`](https://github.com/Instagram/IGListKit/blob/master/README.md) and [documentation](http://instagram.github.io/IGListKit)
- [x] I have searched [existing issues](https://github.com/Instagram/IGListKit/issues) and this is not a duplicate

UICollectionView introduced `prefetchDataSource` in iOS10 that we can ease to do preparing stuffs like image-loading for incoming display cells.

IGListKit disabled the builtin prefetching functionality [due to performance](https://instagram.github.io/IGListKit/Classes/IGListAdapter.html#/c:objc(cs)IGListAdapter(py)collectionView), while it provides work range mechanism for prefetching. It works for sections based adapter like in Instagram main feed in which we place PostSectionControllers, but sucks in Instagram profile feed in which we probably use one IGListBindingSectionController to show user's all photos and one ProfileSectionController to show user's information. If implement it like this, we loose work-range feature to do prefetch jobs because there are only two objects (sections): user-info model and user-array-photos model.

So, does IGListKit consider to add prefetching functionality to IGListBindingSectionController?