Feature request: Support for rendering plantuml with online server

[mokeyish]

If PLANTUML_BIN not set, support for rendering plantuml with online server.

https://www.plantuml.com/plantuml/uml/SyfFKj2rKt3CoKnELR1Io4ZDoSa70000

[tarleb]

Sounds like a good idea. I wouldn't want to make it the default though since I don't want the filter to send user data to remove servers by default.

I don't think I'll have time to implement this in the near future, but PRs are welcome!