Add support for mermaid code blocks

[rubensworks]

With mermaid, you can generate (SVG) diagrams and flowcharts via Markdown code blocks.

We should make it possible to do to something like the following:

 ```svg
graph TD;
  A-->B;
  A-->C;
  B-->D;
  C-->D;
```

One caveat: this library is JavaScript-based, and generates figures client-side.
What we would want is a way to generate SVG images when our article is being compiled, so that no additional JS is needed client-side.
We may be able to achieve this in a similar way as SsKaTeX does server-side KaTeX by running JS internally.