encoding/json: allow per-Encoder/per-Decoder registration of marshal/unmarshal functions

[rsc]

For example, if a user wants to marshal net.IP with custom code, we should provide a way
to do that, probably a method on *Encoder. Similarly for *Decoder.

Same for encoding/xml.

[robpike]

Comment 1:

Labels changed: removed go1.3.

[rsc]

Comment 2:

Labels changed: added go1.3maybe.

[rsc]

Comment 3:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 4:

Labels changed: added repo-main.

[odeke-em]

I did some work on this sometime back in October, with CL https://go-review.googlesource.com/c/31091 to get the conversation started.
Perhaps we can work on it for Go1.9.

In there I introduced Encoder.RegisterEncoder

func (enc *Encoder) RegisterEncoder(t reflect.Type, fn func(interface{}) ([]byte, error))

and Decoder.RegisterDecoder

func (dec *Decoder) RegisterDecoder(t reflect.Type, fn func([]byte) (interface{}, error))