mermaid-diagrams

Enable `Mermaid diagrams` on github wiki and markdown files like the ones found here: http://knsv.github.io/mermaid/index.html.

For example add this to any wiki page or markdown file in github

```mermaid
sequenceDiagram
A->> B: Query
B->> C: Forward query
Note right of C: Thinking...
C->> B: Response
B->> A: Forward response
```

This will generate a nice diagram if loaded from github. The extension will just replace the code block with the generated diagram.