add: support for in-Markdown Mermaid diagrams

[thearchitector]

Includes support and documentation for rendering Mermaid diagrams by default.

Bumps markdown2 to v2.4.8.

I'd also advocate for listing markdown2 as a dependency and pinning the version there instead of vendoring a copy. I think it keeps things neater and in their own wheelhouses; I don't see a reason to treat it differently than the other deps, unless there's a proven track record of incompatibilities with other tools people might use in tandem / I acknowledge it would probably add some dependency maintenance overhead (which might be a valid reason itself).

Closes #524.

[mhils]

Thank you for the very nice PR! 🍰 I've liked this so much that I moved it into the core and added a --mermaid command line flag. 😃

I'd also advocate for listing markdown2 as a dependency and pinning the version there instead of vendoring a copy.

One thing that's special about pdoc is that it gets installed into lots of random virtualenvs with sometimes weird package version constraints. Vendoring makes things easier here. We've repeatedly shipped out-of-band hotfixes for markdown2 (e.g. 912c33d), which is why I'd like to retain that capability. 😃