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.