Support named graphs (via TriG)

[BigBlueHat]

Matches levelgraph/levelgraph-jsonld#54 from levelgraph-jsonld

[mcollina]

yes it would be awesome! Would you like to send a PR?

[BigBlueHat]

Think I'd like to get/see the levelgraph-jsonld one merged (hi there @jmatsushita!) first, so that we can keep the APIs/experience similar. Certainly, I'd want the resulting UX to be the same.

Anyhow. Happy to send a PR once I get the code written. 😉