Provide support for the Mermaid extension

[kghbln]

Currently the Mermaid extension does not support SRF. It will be nice to get this, especially since Mermaid supports Gantt charts.

As to how to best implement (taken from SemanticMediaWiki/Mermaid#1 (comment)):

I'm not planning to integrate this with SRF but the GraphViz printer should provide a good enough bootstrap example for making it work with the mermaid syntax.

[gesinn-it-gea]

@mwjames should integration possibly look comparable to SRF D3:

 | format=mermaid
 | charttype=gantt
 | <other charttype specific parameter>

[kghbln]

I think to have read in some mailing list, that there will be a solution in December. :)

[gesinn-it-gea]

we have implemented a prototype and like to share it with the community during SMWCon Fall 2018. Proposal would be to get it integrated into SRF master during the Hackathon / Tutorial day.

[kghbln]

@RacoonDev has put an effort creating the result format which is awaiting the release of Mermaid 2.0.0 for the pull.

Documented on wiki

[kghbln]

"gantt" was done thanks to @RacoonDev . :) What still could be done is "graph" and "sequence".