SPM support, exposed the delegate method to the public

[wiencheck]

I have added Swift Package Support for this framework and modified the way the delegate is used. I believe it is better to allow any object to adopt the CollectionViewWaterfallLayoutDelegate, not only the object that already conforms to the UICollectionViewDelegate.

I think it allows for better separation of responsibilities and thus cleaner code. Also, creating the layout like this: let layout = CollectionViewWaterfallLayout(delegate: self) is more satisfying ☺️

[JuanpaG94]

This repo seems to be abandoned since 2019, but this addition will be really useful for current implementations @ecerney