Consider PlantUML support

[Alexander-Wilms]

In order to easily add UML diagrams to markdown files, support for PlantUML (some examples: ogom.github.io/draw_uml/plantuml/) would be useful. The markdown code block looks like this:

```plantuml
@startuml
(UML 1.4) -- (Structure Diagrams)
(UML 1.4) -- (Behavior Diagrams) 

(Structure Diagrams) -- (Deploymen/Component Diagram)
(Structure Diagrams) -- (Class Diagram)

(Behavior Diagrams) -- (Use Case Diagram)
(Behavior Diagrams) -- (Activity Diagram)
(Behavior Diagrams) -- (Sequence Diagram)
(Behavior Diagrams) -- (State Diagram)
@enduml
 ` ` `

In order to generate an image from the code, the plantuml.jar and graphviz are required. Calling java -jar ./plantuml diagram.txt results in a diagram.png file being generated.

Cf. https://marketplace.visualstudio.com/items?itemName=jebbs.plantuml for a Visual Studio code extension