This commit introduces a new command-line argument `--output_format`
to `generate.py`, allowing users to specify the desired output format
for generated videos. Supported formats are `mp4` (default) and `gif`.
The video saving logic has been updated to handle both formats, using
`imageio` for GIF generation. The `README.md` has been updated to
document this new feature, and a test case has been added to
`tests/test.sh` (though it was adapted for a dry run due to
environmental limitations during testing).
