Add a decorator which allows to customize the icon of a diagram node

[Tom Roshko]

CDK-Dia currently automatically generates a label for each diagram node according to the resource logical name and cloudformation type.

CDK-Dia also has decorators which allow the user to customize how a construct is diagramed (see main README.md for details).

This issue offers to add such a customizer that allows the user to set a custom icon.