It looks like this library, like gfx and move.js, will accept a cubic-bezier string for easing. I wrote a module which wraps all of Robert Penner's easing equations into cubic-beziers for ease of use:

https://github.com/robdodson/amd-css3-ease

And I talk about it in this blog post, but it's really easy to use: http://robdodson.me/blog/2012/05/27/penner-easing-equations-...

Credit really goes to Matthew Lein who converted the equations to CSS3: http://matthewlein.com/ceaser/