Expose scroll animation value

[loveaurell]

It would be useful to be able to get the animated scroll value. You could implement all sorts of parallax animations as well as the liquid dot animations requested here: #143 . You also would need to use an Animated.Flatlist internally instead of Flatlist. Maybe a prop "useAnimatedFlatlist" could control that?

[loveaurell]

renderItem could receive a param animatedValue that represents an animation from -1 to 1. -1 could represent the previous view, 0 the current and 1 the next.

[pke]

That would be a really useful feature