Support mermaid diagrams

[PhilippMDoerner]

I use mermaid diagrams sometimes, mostly because github markdown natively has support for them.

Recent usage example: https://github.com/PhilippMDoerner/ThreadButler

Nimibook does not render such blocks (since they're not markdown). Recent example: https://philippmdoerner.github.io/ThreadButler/bookCompiled/index.html

I would like for nimibook to also be able to render mermaid-blocks such as this into corresponding diagrams.

Guides for how to do so are defined here: https://mermaid.js.org/config/usage.html?id=usage

Would that be a possible addition?