add support for mermaid.js diagrams

[Mads Schou]

It would be a great feature to enable native support of mermaid.js diagram blocks.
This would enable users to write diagrams as code and keep them inside the markdown files.

It is simple enough to include the mermaid.js dependency and include a raw div with mermaid syntax, but we may need some more tweaking to enable parsing of code blocks.

// Raw div block - these can be picked up by default by mermaid.js just by including the dependency.
<div class="mermaid">
graph LR
    A --- B
    B-->C[fa:fa-ban forbidden]
    B-->D(fa:fa-spinner);
</div>

// Code block - these are not picked up by mermaid.js, since they are rendered as <pre><code> blocks.
```mermaid
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
```

By default the markdown renderer just renders a mermaid block as a standard code block.

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

I dont know if mermaid rendering can be enabled without changing the core zola rendering engine, but it seems like it was achieved by Docsy (a hugo docuementation theme) without changing the core hugo markdown engine, and using jquery instead:
https://github.com/google/docsy/blob/d4b214c8e5c7fabf9e83b58a9df2281be242d5b4/assets/js/mermaid.js

[Dr Alexander Mikhalev]

👍 very useful feature.

[Aaran Xu]

This is a good suggestion. I will add it soon.

[Dr Alexander Mikhalev]

Good. Use case I have in mind diagram with clickable links:

graph LR;

QuickStart(Quick Start) --> CmdPalette(Command<BR>Palette);
QuickStart --> CreateNotes("Create notes");
QuickStart --> InternalLinks("Internal Links");

click CreateNotes "/Create notes";
click CmdPalette "/Command palette";
click InternalLinks "/Internal link";