Consider support for MathJax Configuration

The support for [MathJax Configuration](http://docs.mathjax.org/en/latest/options/index.html) will allow to use additional Latex packages and define custom macroses.

```js
window.MathJax = {
loader: {
load: ['[tex]/tagformat']
},
startup: {
pageReady: () => {
alert('Running MathJax')l;
return MathJax.startup.defaultPageReady();
}
},
tex: {
packages: {'[+]': ['tagformat']},
tagSide: 'left',
macros: {
RR: '{\\bf R}',
bold: ['{\\bf #1}',1]
},
tagformat: {
tag: (n) => '[' + n + ']'
}
}
};
```

The issue names no files, tests, or entry points. Start by locating the extension's existing MathJax integration and determine how configuration is currently passed through. Done should include a decided configuration interface, support for the requested packages and macros, and tests or documented verification for the example configuration.

Scritto dal modello di indicizzazione a partire dal testo della issue.