Introduce Mermaid diagram support in markdown editor/preview

There are many markdown editors (e.g. http://pad.haroopress.com/user.html) that support mermaid diagrams (http://knsv.github.io/mermaid/). Mermaid is a "markdown for diagrams". It supports diagrams: graphs, sequence diagrams, gantt diagrams. Idea is to allow embedding such diagrams right from source markdown file, e.g.

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

This code block shall be rendered into diagram

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