multiple runs may target frames from last run if .ocrvid/frames/ is not cleaned up

[kj-9]

current behavior: saving frames at .ocrvid/frames/${video_name} for each run, without removing last run.

this cause duplication if runs multiple times, changing frames-step option.

[kj-9]

it might be better to not save frames by default?

leave it an option and make users choose. users can save frames if they want, clean up frames after run if they want.

but ensure each run is run on unique frames generated from the input video.
We can use buffer not file to pass to vision framework using initWithData
e.g. here or here

[kj-9]

as side effect, in output json

• .frames_dir is deleted
• .frames_step is deleted
• .frames[].frame_files is deleted, .frames[].frame_index is added instead.

[kj-9]

json in readme is not up to date