Add Mermaid.js Support

[xaprb]

This PR is for adding Mermaid.js support. It's not done. Here's a TODO list:

• Reset default Mermaid styling so the boxes aren't colored? Black and white is fine.
• With Remark, support multiple slides with Mermaid diagrams.
• With Remark and Adirondack, it looks like some but not all of the Mermaid elements get styling from Remark or Adirondack. Some are small Arial-ish, some are large and have the Google fonts from Adirondack.
• With Remark, it would be nice to support .mermaid[...] syntax, but Remark adds a <p> element inside the <div> so it breaks. It might be better to support a fenced code block with the language type set to mermaid instead?
• Perhaps Text in notes displays incorrectly when combining Mermaid and Remark (with example) mermaid-js/mermaid#360 is relevant.

I'll keep tweaking and pushing here.

[xaprb]

I’m sorry but I’m not interested in maintaining this theme anymore. I’m glad it’s been useful, and I encourage someone to continue development elsewhere. The MIT license is very permissive and should not place any burdensome restrictions upon that.