Add media type parameters `profile=` and `version=`

Complete the descriptions of the media type optional parameters.

Modifying/reregistering media types is a cumbersome and disruptive so putting in general optional parameters at this point is simpler.

See, for example https://www.w3.org/TR/rdf12-n-triples/#sec-mediaReg-n-triples, (the `version=` text is specific to RDF and not relevant here).
