Add Support for C4-PlantUML

Thanks to the awesome work done by @dgassma, plantuml-parser now supports C4-PlantUML.

@startuml
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml

Person(personAlias, "Label", "Optional Description")
Container(containerAlias, "Label", "Technology", "Optional Description")
System(systemAlias, "Label", "Optional Description")

Rel(personAlias, containerAlias, "Label", "Optional Technology")
@enduml

[
  {
    "elements": [
      {
        "type_": {
          "source": "Stdlib_C4",
          "name": "Person"
        },
        "alias": "personAlias",
        "label": "Label",
        "descr": "Optional Description"
      },
      {
        "type_": {
          "source": "Stdlib_C4",
          "name": "Container"
        },
        "alias": "containerAlias",
        "label": "Label",
        "techn": "Technology",
        "descr": "Optional Description"
      },
      {
        "type_": {
          "source": "Stdlib_C4",
          "name": "System"
        },
        "alias": "systemAlias",
        "label": "Label",
        "descr": "Optional Description"
      },
      {
        "type_": {
          "source": "Stdlib_C4",
          "name": "Rel"
        },
        "from": "personAlias",
        "to": "containerAlias",
        "label": "Label",
        "techn": "Optional Technology"
      }
    ]
  }
]

Thank you @dgassma for your contribution!