Respect freedesktop's XDG_CONFIG_HOME and XDG_DATA_HOME

Thanks for this tool!

Linux configuration and data storage should reside in XDG_CONFIG_HOME (`~/.config/` if the env variable is not set) and XDG_DATA_HOME (`~/.local/share/` if the env variable is not set). This will provide consistency with other programs. $XDG_CONFIG_HOME/gam/ should house actual configuration files (for instance, gam.cfg, which would have items like `check_updates = false` or `member_max_results = 200`). $XDG_DATA_HOME/gam/ should house the oauth tokens, etc. and should be mode 0700.

Keeping this structure will also allow proper sandboxing when the user wants it.