Add an option to use QOI instead of png

[FrostBird347]

The ​Quite-OK Image format is extremely fast at encoding and thus is best suited for situations where images need to be encoded or decoded as quickly as possible while still using minimal size.
Extracting frames from a video is one such situation, which is exactly what this program does.
FFmpeg already supports qoi, so I don't see any reasons against using qoi instead of png.