jsoncolor does not colorize elements that implement MarshalJSON

[neilotoole]

Currently, if an element implements MarshalJSON, jsoncolor will delegate the byte encoding to that method, and then use those returned bytes. Instead, we should allow the MarshalJSON method to return bytes, and then re-parse those into a v interface{}, which should then be passed to the usual jsoncolor funcs.

[neilotoole]

The same issue affected handling of RawMessage. The PR does handles colorization of both (in practice, the encodeJSONMarshaler gets the JSON bytes and delegates to encodeRawMessage), but with the unfortunate side-effect that keys can be re-ordered in the output.

A preferable solution would be an implementation of encodeRawMessage that decodes the raw tokens and then re-encodes (with indent/color) without marshaling into a map at any time (as the map will lose the key order). This is beyond scope though.