keyboard accessibility

Hello,

Juxtapose is a really neat tool. I was trying it out and I have come up with some suggestions to add keyboard accessibility to make Juxtapose usable by users who don't use a mouse. 

I outlined the purpose of the changes I made in this <a href="https://github.com/radiocontrolled/juxtapose">readme</a>. I hope you can review what I am working on. 

I have been testing this using the example.html file. At this point, the main thing I can't figure out is how to change the focus order on the slider: when I append the handle after the the left image (line 325 juxtapose.js), it breaks the layout. The present focus order gives the slider controller focus, followed by the left and right buttons. To someone navigating with a keyboard a more sequential order (left, handle, right) would probably be more logical.

Thanks,
Alison