Slide numbers don't match URL

[Leftium]

Apparently, the URL is 0-indexed, but the slideNumber starts at 1. It would be less confusing if they used the same numbering system.

Repro steps:

1. Add option slideNumber: true to https://github.com/hakimel/reveal.js/blob/master/demo.html
2. Compare URL/slide number in browser

[bnjmnt4n]

It is true that using the same numbering system is less confusing, however the 0-indexed URL system has been in use since the very first version of Reveal and hence it is unlikely that we will switch to a different system.

[Leftium]

What about changing the displayed slide numbers to also be 0-indexed? It could be added as an additional option for slideNumber to maintain historical behavior.