introduce rdflib as a dependency?

for the more semantic formats, such as DC, schema.org, DCAT it makes sense to offer capability to export them in any of the supported rdf-serialisations; jsonld, xml/rdf or ttl. this capability is offered by the rdflib library. similar for ingesting metadata, the library would be able to ingest data in either one of these serialisations.

```
from rdflib import Graph
g = Graph()
g.parse(data=rdfxml, format='xml')
ttl = g.serialize(format='jsonld')
```

on the other hand it will complicate the setup of this tool...

any thoughts?