XDG base directory support on Linux

[mdebusk]

I'm currently using freac appimage on Ubuntu 20.04.4 and I appreciate your making it available.

Currently freac stores its configuration file and cddb cache in ~/freac. I'd prefer to move them to directories compliant with the XDG Base Directory Specification. Is there a command-line parameter I can use or an environment variable I can set to make this possible?