Add plotting of yaml and json strings

[rkrug]

PlantUML has new diagram types. Of interest, and requiring special treatment, are yaml and json, as these essentially enclose in

@startyaml

THE YAML AS TEXT
@endyaml

and

@startjson

THE JSON AS TEXT
@endjson

A function

plantuml_from_yaml(x)
plantuml_from_json(x)

where x can be either a file name or the actual yaml / json text.

[rkrug]

Added in v0.6.7