[FR] Mermaid diagram support

[efemkay]

Mermaid diagram by default doesn't scale to the screen at all. If its big, it will simply overflow. I want to request feature to limit the diagram to screen size (scaled to width) and use the same toolkit to provide zoom on click similar to how it is for normal picture.

I note that this plugin does support svg (mermaid also output the diagram in svg) but it doesn't work currently. Perhaps related to #68