Add support for custom objects

[SydoxX]

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.