Delta v0.4.0

The library now supports the build-in PlantUML themes
and won't set any colors if a theme declaration is discovered.
This leads to a complete rewrite of the styling declarations
and extending the concept for multi-level declarations.

Changed

• Breaking: Rewrite the styling declaration concept (#9), (@johthor)
• Breaking: Switch activity direction indicator from suffix to prefix (@johthor)

Added

• Add support for PlantUML themes and dark-mode (#9), (@johthor)
• Add style tags to actors, objects and boundaries (#9), (@johthor)
• Add new icon styles for person actors (#4), (@johthor)

Full Changelog: v0.3.1...v0.4.0