Support mermaid syntax and Gantt 

Thanks Jean-Philippe Hamon!

> Hi Romany,
> 
> I’m looking for Markdown App on Mac. So I see on Github and in the Apple Store your App.
> You speek about Diagram support and Tex Expression is for me
> a critical point for the App.
> 
> Is MarkdownD could support mermaid syntax ?
> Gantt ?
> Best Regards.

