Correct use of XDG_CONFIG_HOME

From the [XDG Base Dir Spec](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html):

> $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.

Previously, if $XDG_CONFIG_HOME was not set, the config path defaulted to `$HOME/.config/configstore`. However, if $XDG_CONFIG_HOME was set to a valid value like `$HOME/.config` the path would have expanded to `$HOME/.config/.config/configstore`.