Feature Request: Full Mermaid support

[jessejhein]

In order to use this for code planning and code descriptions I need to enhance this with Mermaid support. (I also plan code syntax highlighting.) I'm doing this on my own branch. The question is if I should upstream it?

First pass I got it working with pure python. However, it was limited and couldn't handle the full modern Mermaid suite of graphs, in particular the class diagrams. And while sequence diagrams are great I don't see a point in Mermaid to have another source of flowcharts, graphviz handled that better. So my second pass I'm reluctantly adding a npm and chromium (headless) reliance so I can use the latest Mermaid.js. This is still in progress so I can't demo the solution.

I'm doing this for my own use. But I would be happy to clean it up and package it for a PR if there is general interest in it.

[MartinPacker]

I can see the attraction of Mermaid. For some reason I thought we had it. In any case...

1. The best syntax is to borrow from RunPython and use a triple backtick bracket with the word mermaid on the same line as the first triple backtick.

2. You might like to experiment with running Mermaid from with RunPython. As a prototyping method if nothing else.

3. This will eventually need some documenting.

4. If there's an external package dependency md2pptx likes to test for its presence and report the level of the package in a message at startup.

5. I wonder if Mermaid can be run independently - though I see the attraction of embedding it: A single text deck (or maybe one with embeds via mdpre).

Just some initial thoughts.