Add support for custom objects

STIX 2.1 allows for custom objects. However, `bundle.Decode()` currently does not allow for them as the decoding of objects is hardcoded with a fallback to a generic `object.STIXObject`. Optimally, one can introduce custom STIX objects into a project and provide a `type` to `DecodeFunc` mapping which in turn `bundle.Decode()` uses to decode custom objects in a STIX bundle.