reveal.js support

[extall]

I've tried to run htmlark on a few reveal.js based presentations (https://github.com/hakimel/reveal.js/).

I found that while in principle it is possible to get results, a lot of features are unsupported and there are many fail cases:

• data-background not supported (loads external images; is specific to reveal.js; should work with data urls);
• math plugin is not woking (maybe because it's an external dependency? I didn't try it with a non-CDN version of MathJax yet);
• fonts are not loaded properly.

Any chance of updating this application adding support for reveal.js? (seems like could be a lot of effort; maybe can collaborate on this.) Anyway, I would like to learn of your thoughts on this. Thanks!