Function for calling relationships() but instead of just returning the
Relationship objects, returns the STIX objects being refered to in those
Relationships.
Don't wrap objects in a Bundle when adding them to a FileSystemStore,
but still support getting objects from FileSystemStore that were saved
in a bundle.
Also:
- Add option to allow custom content in FileSystemStore
- Simplify an if statement
- Improve FileSystem docstrings
- Remove an unnecessary parse() call in FileSystemSource.get()