Better codec control.

[tomchristie]

Allow codecs to be enabled or disabled. And allow a priority to be set for which decoder is used in the accept header.

Might look something like this:

$ coreapi codecs disable corejson
$ coreapi codecs enable openapi --priority

[tomchristie]

I don't think we actually want this. What we will do tho is split out Schema (corejson, openapi) codecs from Client codecs. (json, text, download).

We'll probably want to force users to be explicit about the document type when they load a schema.