Add overlay builder with direction and progress

[jonasblendstrup]

Hello, appreciate your work with this package, especially considering that Appinio Swiper is basically abandoned. I was wondered if it would be possible to add an overlay builder, so that it would be possible to create animated overlays, like Tinder which displays a "Like" or "Nope" text in the corner, when the user swipes (see https://www.virtuallocation.com/images/tinder/tinder-swipe-3.jpg).

Ideally this builder would be a method, that gives the direction the user has swiped, as well as the progress of how fair they've swiped the card from the center. The method would return a widget, that would be drawn ontop of the top card (the one being dragged). The progress would be used to fade in or out the overlay depending on how far the user drags the card, and the direction would be used for showing different overlays, depending on which direction the user drags the card.

The package swipable_stack has this feature, if that is of any help: https://pub.dev/packages/swipable_stack

Thanks again for your work, this package definitely seems like the best and most elegant swipe solution for Flutter.