rdf to-jelly: add an option to emit non-delimited data

[Ostrzyciel]

There should be an option to output a single, non-delimited frame. This is pretty advanced stuff, mostly useful in debugging, testing, development.

[Ostrzyciel]

If we want to keep going through Jena, then we need to pick one option:

• write a custom serializer class for jelly-cli (meh, code duplication)
• switch to Pekko Streams as the underlying stream abstraction (meh? that's a lot of code for just a simple CLI utility)
• add an option to the Jena RIOT writer to do this – probably also to RDF4J's writer

I think I'll go with option 3.