Ability to use encoding.TextUnmarshaler

[john8329]

I think it would be nice to have the library support the standard encoding.TextUnmarshaler interfaces, so when it's about to unmarshal a string into a field (which may be of its own type), it actually instantiates that type and calls its UnmarshalText() method.

This way the object we're unmarshalling to implicitly declares how it can decode data. A use case? UUIDs stored through arrays instead of strings.

An example implementation:

            fi := field.Interface()
_, canTextUnmarshal := fi.(encoding.TextUnmarshaler)
if canTextUnmarshal {
  t := field.Type().Elem()
  newValTyp := reflect.New(t)
  field.Set(newValTyp)

  unmarshaller, _ := field.Interface().(encoding.TextUnmarshaler)
  err := unmarshaller.UnmarshalText([]byte(v.(string)))
  if err != nil {
    panic(err)
  }
} else {
  newVal = reflect.ValueOf(v)
}

The standard go json decoder also implements this if I'm not mistaken.

Opinions?

[mitchellh]

I'd be open to a PR doing this. Thank you!