Keyboard listeners

[martgnz]

Would be great to be able to navigate between steps using the keyboard arrows and the space bar.

graph-scroll has it, and it's quite handy.

[russellsamora]

I don't modify the default keyboard interaction, which still scrolls the page like normal and works fine. It looks like graph scroll hijacks the keyboard events and advances it like a slide. While it is doable, I would prefer to leave that choice up to the developer implementing it on their own, rather than as a default behavior of the library. Thanks for the input though.