Follow XDG Base Directory Specification more closely

[iamthad]

The spec states "$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used."

Instead of using .cider as the default directory when $XDG_CONFIG_HOME is not set, $HOME/.config/cidershould be used.

For full compliance with the spec, we'd also use $HOME/.local/share/cider instead of $/Library/Application Support/com.msanders.cider, but I'd be fine with leaving that as-is, since it is more idiomatic for OS X.

[RogerThiede]

This stackoverflow post suggests that each OS has a different directory standard.

Type	Microsoft Windows	Linux	macOS
Config	APPDATA	XDG_CONFIG_HOME	~/Library/Preferences/
Data	APPDATA	XDG_DATA_HOME	~/Library/
Cache	APPDATA	XDG_CACHE_HOME	~/Library/Caches/