add servlet to encode and decode diagrams

[HeinrichAD]

Encode and decode plantuml diagrams exactly as plantuml does :smile:. This function could also be useful later in the frontend for automatic compilation or rendering of diagrams.

Encode

GET http://localhost/planuml/coder/SyfFKj2rKt3CoKnELR1Io4ZDoSa70000

Result:

@startuml
Bob -> Alice : hello
@enduml

Decode

POST http://localhost/planuml/coder

Body:

@startuml
Bob -> Alice : hello
@enduml

Result:

SyfFKj2rKt3CoKnELR1Io4ZDoSa70000