Support json.Marshaler and json.Unmarshaler

[tejasmanohar]

Solves #12

[achille-roussel]

This works but I'm not sure if this is the right way to solve the problem.
The main issue I'm seeing is that this doesn't produce the same representation than using the standard encoding/json package, for example for this type:

import "github.com/google/uuid"

type A struct {
  ID uuid.UUID // implements json.Marshaler
}

import "encoding/json"
...
u, _ := uuid.Parse("6E78219C-8FD0-49CC-AA29-2B8D87E6D64D")
b, _ := json.Marshal(A{ID: u})
// {"ID":"6E78219C-8FD0-49CC-AA29-2B8D87E6D64D"}

import "github.com/segmentio/objconv/json"
...
u, _ := uuid.Parse("6E78219C-8FD0-49CC-AA29-2B8D87E6D64D")
b, _ := json.Marshal(A{ID: u})
// {"ID":"\"6E78219C-8FD0-49CC-AA29-2B8D87E6D64D\""}

This is a bug that would be pretty hard to track down, I feel like the right thing to do would be to match the behavior of the standard library.

What do you think?

[achille-roussel]

@tejasmanohar any updates on this?

[tejasmanohar]

Sorry, missed this! That doesn't look good... I intended to match behavior of the standard library. Does the problem apply to other standard library encodings in objconv, too (e.g. encoding.TextMarshaler)? I'll have time to investigate later this week.