Implemented XDG basedir spec for local config

(http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html)
Local config is searched for in:
  * $SURFRAW_conf if defined
  * $XDG_CONFIG_HOME/surfraw/conf
  * $HOME/.config/surfraw/conf
  * $HOME/.surfraw.conf (for backwards compatibility)
Removed definition of SURFRAW_conf from global config