Add a flag to compress files in-place

[JinEnMok]

I.e. unpacking, compressing, and then repacking and overwriting the original file, as opposed to writing to a new directory.

[TheHardew]

Please check the update. I still need to update the README though. Usage: python compress_comics.py -o or python compress_comics.py --overwrite

[TheHardew]

It should work, but I didn't want to add it at first since it's risky to overwrite stuff, I don't trust myself to do it perfectly on the first try, so better do it only with things you are prepared to lose.

[JinEnMok]

thanks for the update. I figured you didn't want it to be destructive, but this could be useful for testing or singular converts.