Add option to use prism.js syntax highlighting, as an alternative to highlight.js

[ftessier]

(from: https://help.slides.com/forums/175819-general/suggestions/44767447 and hakimel/reveal.js#3514)

The ability to highlight syntax in code frames is wonderful! It would be even better if one could select alternative highlighting libraries, such as prism.js which offers more languages. For example, I am working on a git presentation, and highlight.js does not offer syntax for git commands or diff output.

Any suggestion on how to accomplish this locally through libs or css will prove useful!

[ftessier]

Reviewed and closed in hakimel/reveal.js#3514