Expose UICollectionViewDataSource methods `indexTitlesForCollectionView:` and `indexPathForIndexTitle:atIndex:`

## 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

### General information

- `IGListKit` version: All
- iOS version(s): All
- CocoaPods/Carthage version: N/A
- Xcode version: N/A
- Devices/Simulators affected: N/A
- Reproducible in the demo project? (Yes/No): N/A
- Related issues: N/A

### Debug information

```bash
# Please include debug logs using the following lldb command:
po [IGListDebugger dump]
```
Hello! I would like to propose a feature to expose the optional methods on `UICollectionViewDataSource` - `indexTitlesForCollectionView:` and `indexPathForIndexTitle:atIndex:`

These methods are used to implement what I would call the 'fast-jump scroll bar' on collection / table views. I'm not sure what it's actually called but a good example of this can be found on the right side of the contacts tab of the iOS contacts app.

Open to any suggestions on the best way to do this, or potential workarounds! Thanks.