Use salt to generate file tree diagram for tree block macro

[mojavelinux]

As an alternative to the currently CSS-based approach, allow the salt diagram from plantuml to be used (via Asciidoctor Diagram) to generate the file tree diagram for the tree block macro.

See https://github.com/asciidoctor/asciidoctor-diagram/blob/master/lib/asciidoctor-diagram/plantuml/extension.rb#L70