Show is stateless now

With this update, we add a parameter to all slide functions named ctx, of type Context. This object now contains all the layout logic of the slideshow, so instead of calling show.markdown you will now call ctx.markdown. This allows us to completely detach the show object from the state of each slide. So now every slide is stateless, animation status and text input state is maintained in JavaScript, and it becomes safe to run a slideshow on different browsers.