jskos-convert: support RDF input format

[nichtich]

Together with #12 to replace Perl script skos2jskos. Requires a triple store so maybe better as an independent package jskos-rdf.

[nichtich]

In addition to RDF/SKOS we would also like to convert OWL and RDFS ontologies to JSKOS format. The semantics of conversion is simple, e.g.

• owl:Ontology => JSKOS Concept Scheme
• owl:Class => JSKOS Concept
• rdfs:subClassOf => broader
• owl:domain, owl:range (if another class) => related
• ...