flags: use $XDG_CONFIG_DIRS and $XDG_CONFIG_HOME as defaults

This does not add support for '$XDG_CONFIG_DIRS' expansion, it just makes the
default include what it is set to.
docs: Update change log with XDG_CONFIG_DIRS change
commands: Remove print statement
docs: Fix typo in changelog
Merge pull request #615 from adamcik/feature/xdg_config_dirs

Add $XDG_CONFIG_DIRS to default config search path.