URL changes after the whole transition animation is finished

This happens together with [mdpress](https://github.com/egonSchiele/mdpress/), I don’t know if that’s the case with all impress.js presentations.

I would be way snappier if the URL changed as soon as you hit a key for the next slide.

Reproduce the behavior with an mdpress presentation and a keyboard transition, then inspect the navigation and URL-update handling in impress.js. Done means the URL changes when the next-slide key is pressed, rather than after the transition animation completes.

Written by the indexing model from the issue text.