Add support for Mermaid

[glesica]

It would be cool to be able to output diagrams to Mermaid format since that tool is quite popular / useful.

See: http://mermaid-js.github.io

[seattlesbest]

Although plantuml is a very good system, it requires a java or dot environment, and every time the system developers made changes to the data tables or added functions, they had to worry about converting plantuml to png or svg and then further reflecting the changes in a word document. In other words, they found the plantuml format inconvenient to incorporate into explanatory materials for managers. The mermaid format also supports the insertion of drwa.io's XXX.drawio.svg and YYYYY.drawio.png, so explanatory materials can be unified in md, and if glesica can make dcdg mermaid-ready, it will definitely make a world of difference. I am very much looking forward to it, and although it's been almost 4 months now, I can't sleep at night every day thinking about the future where the "-b mermaid" option will become a reality.

[plumsky]

@glesica When this https://github.com/glesica/dcdg.dart/pull/56 will be merge to master?

[glesica]

Once I've had time to address the outstanding issues described in the PR.

[bperreault]

I took this branch that has the mermaid option. It was very useful in diagramming my mobile app. I found there were a few lines that I had to remove from the generated code to allow the graph to be displayed but overall excellent to have this. Thank you.

[glesica]

This is done.