Add the mermaid parser

**Is your feature request related to a problem? Please describe.**
I want to use diagrams with MicroPad

**Describe the solution you'd like**
A way to insert [Mermaid](https://mermaidjs.github.io/) diagrams (possibly new element type)

**Describe alternatives you've considered**
This could be implemented in the parser if we want to make it an NPX feature.

Raised by #94 