Add Unique URIs for N-Quads, TriG and RDF/JSON

The current list of Unique URIs for RDF Formats [1] does not include URIs
for N-Quads [2], TriG [3] or RDF/JSON [4]. All of these have well-known
unique media-types and recommended file extensions and so could easily have
descriptions created in a similar way to the other formats.

The W3C editors draft for TriG [5] attempted to synchronize it with Turtle,
removing its unique media-type and file extension, but it isn't included in
the current Turtle draft so people are still supporting the original
specification

Thanks,

Peter

[1] http://www.w3.org/ns/formats/
[2] http://www.w3.org/TR/n-quads/
[3] http://wifo5-03.informatik.uni-mannheim.de/bizer/trig/
[4] https://github.com/iand/rdf-json
[5] http://www.w3.org/2010/01/Turtle/Trig