* Add a note for Homebrew users.
* Fix possibly unbound variables.
* Remove unused variable.
* Improve default model file lookup; minor style fixes.
* [libvmaf] use vmaf_models/vmaf_v0.6.1.json by default.
* Include the model files.
* Add model files.
* Detect whether used ffmpeg is from Homebrew.
* Add placeholder and instructions for model path, addresses 19.
* Change n_threads to int, minor fixes.
* Remove program name.
* Remove a duplicate line.
* No need to include the default value in the help msg as ArgumentDefaultsHelpFormatter takes care of this.
* Add option to set the value of libvmaf's n_threads option.