Feature request: deserialize to/from raw json

[reinux]

In relation to #74 , there needs to be a way to deserialize json to raw json for further parsing later.

For example, Home Assistant returns json objects that look like this:

{
   "id": 18,
   "type":"event",
   "event":{
      "data":{
         "entity_id":"light.bed_light",
         "new_state":{ ... },
         "old_state":{ ... }
      },
      "event_type":"state_changed",
      "time_fired":"2016-11-26T01:37:24.265429+00:00",
      "origin":"LOCAL",
      "context": {
         "id": "326ef27d19415c60c492fe330945f954",
         "parent_id": null,
         "user_id": "31ddb597e03147118cf8d2f8fbea5553"
      }
   }
}

The sub-type of the event can't be parsed using JsonUnion unless I can deserialize it separately of the message type.