(matrix-org/matrix-spec-proposals#2191)

Firstly, this implements a commonmark-java plugin which is solely used to parse
LaTeX input in the composer box, so that they can be rendered into
`<span data-mx-maths=...>fallback</span>` and `<div
data-mx-maths=...>fallback</div>` for inline and display maths
respectively in the sent message.

Secondly, received messages of this form are pre-processed by a simple
regex into a form which markwon (which performs the rendering) expects.