plugins/chat: use connection id for target instead of object_id

This allows us to look up the connection IDs of familiars and
DM-possessed creatures, which would otherwise not have been possible,
since GetID on nwnx side returns OBJECT_INVALID for player objects
currently possessing something.
Merge pull request #16 from niv/pr-chat-fixes

plugins/chat: use connection id for target instead of object_id