Add modifier for nested type codec constructor

[hyeonjae]

If the record has avro type, the codec is not recognized.

{
  "type": "record",
  "name": "Parent",                  
  "fields" : [
    {"name": "child", "type": "Child"}
  ]
}

To solve this problem, I suggest the following constructor modifier

parent := `
{
  "type": "record",
  "name": "Parent",                  
  "fields" : [
    {"name": "child", "type": "Child"}
  ]
}
`

modifier := func(st map[string]*Codec) {
	st["Child"] = childCodec
}

codec, err := NewCodec(parent, modifier)

As NewCodec function prototype is like below, doesn't hurt compatibility.

func NewCodec(schemaSpecification string, modifiers ...CodecModifier) (*Codec, error) 

[xmcqueen]

@hyeonjae Can you provide a fail case so I can understand the problem a little more? I was toying with making that a variadic and adding a codec just the other day for a different purpose. It would be ideal to get a test case that fails. I'll look into this a bit more now that I see you've already provided some test cases. Thanks

[xmcqueen]

I like this idea. I'll wait a bit for folks to comment.

[survivorbat]

I like this idea too :)