Update config location to use XDG standard

[StewartThomson]

The CLI should place its configuration file according to the XDG Specification.

See here for more details on the XDG Specification: https://wiki.archlinux.org/index.php/XDG_Base_Directory

If we're okay with the dependency, https://github.com/adrg/xdg looks good & robust for handling this cross-platform.

Let me know if you're interested, and I'll get to work on it.

[jmattheis]

This would be ok, as long as old config paths will still be supported.