Support XDG Base Directory Specification #1316

[razor-x]

• If $XDG_CONFIG_HOME is set, config file should be saved to $XDG_CONFIG_HOME/gr/config.json.
• In addition or instead of the above, read GR_CONFIG_HOME and if set, use that (GR_CONFIG_HOME=$HOME would replicate current behaviour).

See https://wiki.archlinux.org/index.php/XDG_Base_Directory_support

For other projects struggling with this, see https://www.google.com/search?q=support+xdg+github&oq=support+xdg+github