config.py: Look for global config files and use XDG dirs

orpsoc now tries to read from /etc/orpsoc/orpsoc.conf,
$XDG_CONFIG_HOME/orpsoc/orpsoc.conf (XDG_CONFIG_HOME defaults to ~/.config)
and finally orpsoc.conf in cwd. Options in each file overwrite the previous
value. Cache location now defaults to $XDG_CACHE_HOME/orpsoc
(XDG_CACHE_HOME defaults to ~/.cache) unless explicitly overridden with
cache_root in any of the config files