Support the asciidoctor-diagram extension

[hallvard]

Asciidoctor supports diagrams using the block syntax, e.g.
[plantuml]
....
here is the PlantUML diagram code
....
But this is not supported by the asciidoctor extension's backend.

If you install the asciidoctor-diagram extension in the docker file using "gem install asciidoctor-diagram" and use the require option (-r): -r asciidoctor-diagram in the shell call, diagram images should be generated (in the same folder as html file) and could be served by the backend.

[hallvard]

I've verified that if I use "gem install asciidoctor" and run asciidoctor with the -r asciidoctor-diagram option in the gitpod/Theia terminal, diagrams are rendered.

[hallvard]

Note that the rendering slows down considerably, so it may be nice to turn the -r option off, or have a button on the preview for explicitly requesting a full render with diagram.