iframe background preload behavior now matches inline iframes + adheres to the new 'preloadIframes' config option

Ref. #1856, #2006, #2045.

This is a more open approach to allow customisation of all MathJax options instead of select options only.

Windows carriage return line endings were not being tested for.

Fixes #2087.

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