XDG Base Directory Compatibility

Would the project be amenable to being [XDG compatible](https://wiki.archlinux.org/title/XDG_Base_Directory) instead of defaulting to `~/.passage`? That is, first check to see if the `XDG_CONFIG_HOME` environment variable is defined, and if so, put `.passage/` under there. In the typical configuration, this would be `~/.config/passage`.

This simplifies my backups, as I can copy `~/.config/` without having to special case dot folders under $HOME.