Support Schema.org style ontologies using Classe and Property items

[ptsefton]

It would be good to be able to support the way Schema.org defines terms - here's an example in RO-Crate format (though there is no link from the root dataset to the Class and Property definitions yet)

https://github.com/Language-Research-Technology/language-data-node-tools/blob/master/ontology/ro-crate-metadata.json