Add mermaid export

[Alex Garel]

https://github.com/mermaid-js/mermaid is a project which is interesting to integrate charts in a lot of support (for example a lot of wiki and documentation tools supports it).

It supports building mindmaps (see also this example).

Supporting mermaid export of mindmaps would be great ! (with the warning that it would be a partial export, as it is for text).

[Paulo Veiga]

Hey. It really great idea. Added for future evaluation.

[Marcel Ackermann]

Came here to create a feature request for this. So +1