Implement site-wide configuration settings

* add site_config_dirs() to appdirs to determine locations across OSes
* add system_config_files to locations.py
* add system_config_files to get_config_files() and re-order files entries to correct precedence
* document changes to configuration files in user guide

Closes #309
Well, if they are attempting to use the XDG directories but
didn't configure it explicitly. It doesn't cost anything to
check it by default and actually makes it easier to explain.