Add image cleanup support

[Drewsif]

I would like to add an option so that users can point to a script to be run to help cleanup images. I do not want to add any code into PiShrink to do the actual cleanup but still support users who want to do it.

My initial thought is to have an option to call a user provided script. After changing the working directory to the mounted image, and possibly pass in the directory as the only parameter. This would allow users to use relative paths or use the argument to clean up their image as they see fit.