Support XDG Base Directory Specification for custom image folder

[jaketmp]

The XDG Base Directory Specification provides a bunch of folders for collating config files, and mechanisms to override the default (~/.config), so our home folders don't end up stuffed with dot files.

It would be great if AnyBar could support .config/AnyBar in addition to .AnyBar.

[tonsky]

Do you want to provide a PR?