Add ability to model subPropertyOf

[gmcfall]

The "RDF Schema" plugin to StarUML exports RDF properties, but there is no way to specify that a given property is an rdfs:subPropertyOf some other property. Can you add such a feature?

Ideally, I would like to use an attachment to specify this relationship. For example, I would like to add an attachment of the form

subPropertyOf=<anyURI>

To any UMLAttribute or UMLAssociationEnd.