RevealMath.MathJax3: Allow the skipHtmlTags option to be overriden

replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

The previous logic here was backwards, and did not allow the user to override `options` in the mathjax config structure.
This makes it match how the `startup` and `tex` fields are merged.

Fix DOM XSS

fix fullscreen issues; correct size in Safari, auto-focus presentation when entering fullscreen #3080

Fixing background video not playing on certain small factor devices (android)

fix npm start -- --host=0.0.0.0 unuseful