[FEATURE REQUEST] Add to Mermaid support to current Flowcharts and Sequence Diagrams #389

This Markdown app http://support.typora.io/Draw-Diagrams-With-Markdown/ shows how to create Sequence Diagrams and Flowcharts in Markdown as well as another type called Mermaid. It uses the same libraries that Boostnote uses:

• js-sequnce https://bramp.github.io/js-sequence-diagrams/
• flowchart.js http://flowchart.js.org/

The 3rd type Mernaid has a JS library they use here https://knsv.github.io/mermaid/#mermaid

The reason I request Mermaid support be added as well...

• It allows to do if else flowcharts which any use I would have for a chart like this would require me describing if else type actions to show the flow of an app.
• It also has a loop UI element to show a loop iteration in the diagram/flowchart
• It also allows to build a Gantt Chart