Load exactly one configuration file, searching XDG_CONFIG_DIRS

Closed Johann Dreo requested to merge github/fork/smcv/xdgconfig into develop

Created by: smcv

~/.liquidpromptrc is still the highest-priority, and /etc/liquidpromptrc is still the lowest.

Rebased on develop, which already looks in the correct $XDG_CONFIG_HOME (not $XDG_HOME_DIR like master did), but does not do the full path-search behaviour described in the basedir spec.

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