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

[Malecks]

New issue checklist

• [X] I have reviewed the README and documentation
• [X] I have searched existing 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

# 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.

[zhubofei]

@rnystrom Shall we add this feature to 4.0?

[rnystrom]

@zhubofei happy to accept it! Doesn’t have to be crammed into 4.0, but can merge it whenever. Seems like a great feature.

_{Sent with GitHawk}

[kronik]

@rnystrom is it already available?

[mohpor]

Badly need this one.

[lorixx]

@mohpor Feel free to add a new PR for this! :D