support mermaid diagram blocks

[erikerlandson]

It would be cool to render mermaid diagrams, like github markdown:
https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams#creating-mermaid-diagrams

[jenshalm]

Agreed, would be nice to have. It's too late for 0.19.0 which is nearing its completion, but I'll add it for the 0.19.1 milestone as there is probably a binary-compatible way to implement it.

[odenzo]

Or PlantUML since that may be more aligned as it has a Java Library

[jenshalm]

Completed in #516