Some little improvements

Hi, I've created some improvements to your impress.js-fork:
* Added the ability to toggle a blank screen with `.`, just like PowerPoint can.
* Added the ability to navigate backwards with `backspace`.
* Now the last substep gets deactivated when navigating to the next step.

I hope you want to pull, so more people can benefit. I chose to fork from you instead of bartaz, since I do like the substeps a lot.