Convert image format/postprocessing

[Efreak]

A large number of older webcomics have large png and jpg files (some nicely optimized, some not). It would be nice to have an alternative for long-term archiving, to save space, such as converting files to webp automatically, as they're downloaded. Currently I'm using cbxconverter, but that's a windows only gui program that (at least on my laptop) takes longer to process each image than it takes to download it and can only be run after the comic has been zipped, not while it downloads.