Preprocessor

[Enteee]

Is your feature request related to a problem? Please describe.
PlantUML comes with a built-in preprocessor official documentation. In order support parsing of all different PlantUML diagrams, this parser should also implement preprocessor instruction parsing and a JavaScript implemented preprocessor (preprocess()).

Describe the solution you'd like

• Add test fixtures for all examples in the official documentation
• Create a new parser for preprocessor instructions
• Add those preprocessor instruction set to the plantuml parser -> make the normal parser skip over preprocessor instructions.
• Write preprocess(diagram: string): string which runs all preprocessor instructions on a raw diagram

Describe alternatives you've considered

[Enteee]

related: #25