feature: cli exit code

[aivarsV]

Return code could be useful for scripting. For example, I always keep only one image file in my wallpapers folder. If download was succesful, I would like to remove previous wallpaper, but I have no way of knowing if new image was actually downloaded.

In theory I could write some hacky script, that checks, if file count in folder has increased, but exit code seems more elegant solution.