Feature Request: Metadata when decoding into maps

[EronWright]

I would like to be able to decode to a map, with deep processing of the data, and collect metadata about the keys that were decoded. For example:

actual := map[string]interface{}
config := &mapstructure.DecoderConfig{
  Metadata:   &mapstructure.Metadata{},
  Result:     actual,
}
decoder, err := mapstructure.NewDecoder(config)

decoder.Decode(map[string]interface{}{
  "a": map[string]interface{}{
    "b": "value",
  },
})
assert.Equals(t,  []string{"[a]", "[a][b]"}, config.Metadata.Keys)

At the moment, the nested map (a) is simply referred to directly in the target map, and the .Metadata.Keys contains only [a].