expose getPastSlideCount() via public API

This can be used by plugins which are concerned with helping the
presenter with their pace.

README.md

@@ -495,6 +495,7 @@ Reveal.getPreviousSlide();
 Reveal.getCurrentSlide();
 
 Reveal.getIndices(); // { h: 0, v: 0 } }
+Reveal.getPastSlideCount();
 Reveal.getProgress(); // (0 == first slide, 1 == last slide)
 Reveal.getTotalSlides();