cdn.mathjax.org is shutting down: https://www.mathjax.org/cdn-shutting-down/

This changes the default MathJax URL in the math plugin, as well as
references in README.md and test/examples/math.html

Closes #1163, #1495.

If we:
- visit a vertical slide

and then:
- go to the first slide

the function will return **false**.

Added a test to isLastSlide too just to be sure.

renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes

- Reformat
- Use new default separator format: '.element: x=y'
- Add new tests for testing that default format

- Reformat.
- Change default separator test 'slide: x=y'