respect the XDG_CONFIG_HOME environment variable

I prefer to have my supernova config file located at ~/.config/supernova.  This patch will allow a user to set the environment variable XDG_CONFIG_HOME to define a location for the config file.  This variable is part of the freedesktop.org standard.

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

"There is a single base directory relative to which user-specific configuration files should be written. This directory is defined by the environment variable $XDG_CONFIG_HOME."