Setting vertical slide to the first or last when navigating to the next or previous slide

Navigation to the next or previous slide (with Page Up/Down) seemed a bit odd to me, because it wasn't preserving the natural order of vertical slides when moving between horizontal slides.

I expected that when I navigate to the next horizontal slide which includes multiple vertical ones, then I should always move to the first vertical slide. In turn, when navigating to the previous horizontal slide which includes multiple vertical ones, I expected moving to the last vertical slide.

That was, however, not always the case. This commit makes reveal.js behave as I described above.