Don't create new slide on pauses

[mfontanini]

Before this change, a <!-- pause --> would copy the contents of the slide so far, create a new slide, and continue adding content to the current slide. This meant that every pause operation would create a new slide, which has the side effect that the footer's current_slide variable changes every time you jump in between a slide's pauses.

With this change a slide is now partitioned into chunks. All slides have at least 1 chunk and every pause creates a new chunk. So this means a slide with 2 pause operations has 3 chunks: what's before the first pause, what's between the first and second, and what's after. This requires dealing with jumps between slides a little different so that you jump through chunks first and then through slides.

Fixes #16