expose getPastSlideCount() via public API

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

js/reveal.js

@@ -4975,6 +4975,9 @@
 		getState: getState,
 		setState: setState,
 
+		// Presentation progress
+		getSlidePastCount: getSlidePastCount,
+
 		// Presentation progress on range of 0-1
 		getProgress: getProgress,