Support for new Composer Decorators

Adds support for Composer's new decorator syntax in .cto models.

Tweaks the PlantUML diagram generation to add in support for the "blue" theme diagrams. This is made available with the following syntax in settings:

"composer.UML.diagramTheme": "blue"

If you are fed up with the default "yellow" diagrams you should give this a try. Also, if you think "blue" should be the default for diagrams in a future release, write me a review and let me know :-)

Note that all PlantUML options are now specified using "composer.UML.xxx" syntax.

Enjoy!