Options to MathJax extensions

[rbeezer]

We load MathJax extensions via the dead-simple, purpose-built, \require{} macro.

But they can be loaded from within the MathJax configuration. And then you can specify options to the extension. Not sure if we need this or not. Unclear if the configuration is a better place that using the \require{} macro.

But witha new specification at 16ab3b7 we could add an attribute like @mathjax-options that would be a string in JSON-ish format giving these.