Adds the ability to access a Node's tag during decoding

[ssgutierrez42]

Summary

Enables access of a Node's tag:
e.g. !DanceEvent

structure:
  version: 0.1.0
  actions:
    8a139ac7: !DanceEvent
      type: happy

During the decoding function of Codable objects:

 required init(from decoder: Decoder) throws {
      ...
      print("\(decoder.tag)") //!DanceEvent
      ...

Background

See #265

Enables an avenue to decode polymorphic types.