Add support for JSON:API

[volmedo]

Hi all, first of all I'd like to thank the go-openapi devs for the great set of tools. They, along with go-swagger, are quite handy and save a lot of time.

I'm using go-swagger to generate server and client code for an API that uses the JSON:API spec to format its JSON messages.

I'm aware of #32 and #33 and understand that inferring consumers/producers dynamically from the spec is not an obvious change. While such a mechanism is implemented, I'd like to propose to add support for JSON:API MIME Type.

The popularity of the JSON:API spec is increasing and adding support for it should be trivial since the serialisation format is still JSON. AFAIU, after a new constant for the application/vnd.api+json is added, the only other thing to do would be to associate the constant to JSON Consumer and Producer in the default runtime's Consumers and Producers maps, respectively.

If you think this makes sense I can do the changes and create the PR myself if you want.