Add MathML elements to whitelist

[alexmilowski]

In looking at lib/html/pipeline/sanitization_filter.rb, I see that all the MathML elements are missing from the whitelist. Presentational MathML is certainly innocuous markup. It could be added to this list without having harmful side effects.

Meanwhile, MathML is part of HTML5 and it is essential for creating accessible web pages with embedded Mathematics.

I'd be happy to provide a list of elements to add to the whitelist.

[gjtorikian]

Sounds good to me.

[fred-wang]

I recommend relying on https://mathml-refresh.github.io/mathml-core/#mathml-elements-and-attributes

[alexmilowski]

I should just write a PR for this.

[gjtorikian]

A new (beta) release of HTML-Proofer has been released, v3.0.0.pre1. I tried to go back and address all the issues in this repo. For this specific issue, I've opted to keep the sanitization process the same. However, downstream consumers can easily make any change they want to the sanitization process; see the updated README for more information.