Introduce Mermaid diagram support


#1

example:

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

This code block shall be rendered into diagram

image

That is, Hugo not hightlight mermaid block, only transform the block to mermaid syntax, then mermaid.js can parses and renders it.

  <div class="mermaid">
  graph LR
      A --- B
      B-->C[fa:fa-ban forbidden]
      B-->D(fa:fa-spinner);
  </div>

This is very useful feature for documenting code/high level design/architecture.