Write a browser extension to convert "HTML Math" for chemistry into MathML

This isn't really a MathCAT project, but it would open up more content to be available for MathCAT and I don't know where else to put it.

Currently there is an [accessible-wikipedia-math](https://github.com/rsoiffer/Accessible-Wikipedia-Math) extension that reverse engineers the HTML math used in Wikipedia for some inline math. That makes the inline math accessible (correct speech and correct braille). The idea here is to either add to that extension or create a new one. For example, $[\mathrm{SO}_4]^{2-}$ is encoded as:
```
[SO
<sub>4</sub>
]
<sup>2−</sup>

```
The classes above (and maybe some others) can be searched for and turned back into MathML's `mmultscripts` tag.

Note: for chemistry, `mmultscripts` should be used and not `msub`, etc., because `mmultscripts` will get the sub and superscript alignments right across multiple sub/superscripts.

This is a good project for someone who knows JavaScript because it is very similar to the math extension mentioned above.