Method to Get Old Index Path for New Index Path in Update

In https://github.com/TextureGroup/Texture we're looking to support using NodeControllers for each item, and having the NodeController survive across object updates (`reloadItemsAtIndexPaths:`) just like IGListSectionControllers do.

Since IGListKit submits reloads as delete/insert pairs, I can't see any public way for us to find out whether, in a given batch update, an insert corresponds to a delete – if an item was updated.

@rnystrom @jessesquires Would you be willing to support that or point me in the right direction?