Handling media type parameters

[mattgarrish]

Not a critical issue we need to solve now, but by adding a parameter for opus it raises the question of whether paramaters are allowed on any media types and whether the full syntax is allowed, or do core media types have to exactly match their representations in the table in 3.2

As it hasn't presented itself as a problem to date, though, I'm going to defer this issue and we can take it up after 3.3 if we think it needs addressing.