Use XDG Base Directories

[PythonNut]

It would be nice if outrun could store its files in the standard locations.

• The outrun config would be stored in $XDG_CONFIG_HOME so the config file would typically be ~/.config/outrun.conf.
• The outrun cache would be stored in $XDG_CACHE_HOME so the cache would typically be ~/.cache/outrun.

References:

• Arch Wiki entry
• Freedesktop specification