Exposes `UIScrollViewDelegate` to clients

[ruddfawcett]

Have you read the Contributing Guidelines? 🖤

Closes #131

Describe your changes

• Allows clients to set a weak var scrollViewDelegate: UIScrollViewDelegate? on CollectionViewDriver
• Implement the UIScrollViewDelegate methods in CollectionViewDriver and forwards them to this delegate
  • Keeps the method signatures in the same order and groupings as UIScrollViewDelegate

Example

• Adds example to ListViewController, demonstrating an override of scrollViewShouldScrollToTop(scrollView:)

Before	After

Todo

• Add unit tests.

[jessesquires]

Thank you so much @ruddfawcett! I will take a look at this very soon.

[ruddfawcett]

Happy to help @jessesquires; I grew up on IGListKit, so really the least I can do especially when your spec was so clear haha. Apologies for lack of tests—added it as a TODO but got distracted by something I was working on.

[jessesquires]

@ruddfawcett no worries at all! 😄

[jessesquires]

Sent an invite to make you a collaborator! 😊