Add maintainContainerHeight prop
As discussed in #80, FlipMove removes items that are leaving from the layout calculations, making its container "jump up" immediately.
While the ideal behaviour is to smoothly animate to the new height, such animations are not possible using hardware-accelerated CSS properties.
This next-best-thing prop allows the container to remain open until the animation is completed, which feels much more intentional (and much less buggy) than the container shrinking immediately.
Thanks to @rhys-vdw for contributing this feature!