Callback to trigger prior to animation
`window.addEventListener('push', myFunction);` is a callback which will trigger after animation. I would like to trigger a function (specifically riot.mount from riotjs) before the animation though after the swap at https://github.com/twbs/ratchet/blob/master/dist/js/ratchet.js#L577, providing cleaner page transitions.

I can of course just edit my version of ratchet, though I'd like ratchet to have a generic solution.
