This shouldn't make a big difference in the CLI/extension
usage, but might improve the performances of long-running
instances, or people misusing the API.
- This commit introduces optional dependencies (namely ffmpeg):
mat2 will spit a warning when trying to process an .avi file
if ffmpeg isn't installed.
- Since metadata are obtained via exiftool, this commit
also refactors a bit our exfitool wrapper.