Feature Request: UML Diagrams

[matbrgz]

Render UML diagrams using Mermaid. For example, this will produce a sequence diagram:

sequenceDiagram
Alice ->> Bob: Hello Bob, how are you?
Bob-->>John: How about you John?
Bob--x Alice: I am good thanks!
Bob-x John: I am good thanks!
Note right of John: Bob thinks a long<br/>long time, so long<br/>that the text does<br/>not fit on a row.

Bob-->Alice: Checking with John...
Alice->John: Yes... John, how are you?

[damageboy]

FYI, This is a dup of:
#1364