Add a 'recursive' flag to .add_artifact() so that directories can be more easily saved

[decodyng]

• Add a 'recursive' flag to the add_artifact() method
• Create logic within FileStorageObserver for saving a directory
• Create an artifact_directory_event in FSO and ties the calling of that event to add_artifact being called with a recursive flag
• If a recursive flag is attempted to be used on an observer that doesn't implement artifact_directory_event, a warning is thrown