[FEATURE] CLI should output to output/

[jesse-wei]

GUI already does this.

Need only use GUI/main.py export_json() and/or export_curr_slice_as_img() in CLI.

Preferably, these functions should be moved to some file in utils/ so that they can be accessed from both CLI and GUI.

As an aside, a lot of other code in CLI and GUI should be factored out to file(s) in utils/.