Add option for separate transitionIn/transitionOut animations

Or more precisely: A transition should be mapped between two slides:

Say you have slide "A:Fade" and slide "B:Zoom". So if one changes from A to B, it would be nice to have A zooming out and B zooming in. Instead of A fading out and B zooming in.

Does this make sense? I'm happy to make my coder hands dirty and help if someone gives me some advice.