Focus on config/preferences only

[Andy Barron]

This lib should focus exclusively on config/preferences. Changes:

• Switch to $XDG_CONFIG_HOME with default to ~/.config on *nix platforms.
• Auto-suffix config files with .cfg (or .cfg.json, or something else entirely) to avoid naming conflicts on systems that don't specify different locations for e.g. config and data
• Copy path logic to a new crate modeled after appdirs
• Remove path logic from preferences-rs and depend on the new crate