Adds support for diagrams extension:
Using a fenced code block with the mermaid
language info will output a <div class='mermaid'>
instead of a pre/code
block:
```mermaid
graph TD;
A-->B;
A-->C;
B-->D;
C-->D;
```
.
<div class="mermaid">graph TD;
A-->B;
A-->C;
B-->D;
C-->D;
</div>
TODO: Add other text diagram languages