Graphviz extension for Showdown.

• download and include Viz.js
• download showdown-viz.js and include it to the html
• registrate the extension when initializing the converter, like

var converter = new showdown.Converter({extensions: ['showdown-toc','showdown-viz']});

• Use markdown code block with tag dot-parse, e.g.

```dot-parse

// your Graphviz code here

```

then the <pre> element generated by the code block will be replaced by the corresponded svg element.