Add interactive slideshow start/stop

I love this project!

I wanted the ability to toggle the slideshow with the spacebar, so I added that functionality. I also added a default interval for this, moved the auto slideshow functionality to an autoSlideshow option, and added a different slideshow transition speed. (I find it nice to have a smooth slideshow transition, but be able to quickly jump back and forth if desired)

Thank you for all your hard work.