Handle other plantuml syntax for generalization

[Aran30]

Currenlty, the PlantUML to BUML converter understands the PlantUML syntax " A <|-- B " for generalizations. Yet, another way of doing that would be in the class definition itself: class A extends B {...} This is currently not be supported by the converter but I feel might be relevant and seems not too difficult to add.

[jcabot]

We are now reconsidering the "parsing" approach underlying new syntaxes around BUML so this will need to wait for that but I agree, it's a small but nice improvement