Upgrade to v4

Closes #1

See merge request !2

Correct a comment

Call me crazy, but this seems like a typo

Removed incorrect possessive apostrophe

gulp: fixing base path for 'package' task

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