MathML support

[MathML](https://www.w3.org/Math/whatIsMathML.html) is the next generation of math display for the web. There are several tools for [converting](https://developer.mozilla.org/en-US/docs/Web/MathML/Authoring) Latex code to MathML code, but there are also [other languages](http://asciimath.org/) that can convert to MathML. I am wondering if the parsing and rendering engines could be separated out so we can be agnostic about the input language, process an abstract math element, and render into Makie.