Region transition animations

[caseycesari]

We need a way to animation transitions of regions.

Baked-in support for region transitions will be part of the v3 release, but it looks like that is a ways off.

This is the best approach I could find, from one of the Marionette contributors: https://github.com/jmeas/marionette.transition-region. It's a prototype though, and maybe not suitable for production.

Here's a lengthy discussion of the issue with some other suggested approaches.