Add MermaidJS support

Go from Markdown defining Mermaid syntax to an SVG

Yes Yes definitely this would be awesome.

A few related things:

https://codepen.io/netsi1964/pen/MWMboow
https://github.com/mermaid-js/mermaid-cli
https://mermaid.ai/open-source/adding-new-shape.html

Seems possible.