Feature Request: Adhering to xdg-base-dir standard

[mqus]

I just found out about your tool and it seems to be just the tool I missed for a while, I have only one gripe with it: It forces me to put another directory in my already-bloated home directory. Therefore I suggest to make use of the XDG Base Directory Standard (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html#basics) to locate for the customizepkg directory. For backwards-compatibility we could still look into the old directories, but I would really like it if we have the standard-conforming paths as default in the README.

For example, without setting $XDG_CONFIG_HOME, the files would now be expected to be in ~/.config/customizepkg instead of ~/.customizepkg. For the global configuration, if $XDG_CONFIG_DIRS is unset, use /etc/xdg/customizepkg.d instead of /etc/customizepkg. But to be honest, I think we can let the default for the global directory stay the same and simply use $XDG_CONFIG_DIRS if it's set.

This has the advantage of making a backup of all user configuration files (of all programs using the xdg-standard) simpler than listing all folders in ~ which should be backed up. This also makes it configurable for users who have their own idea of where config files should go.