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
I guess just in the case that someone *does* have XDG_CONFIG_DIRS but doesn't have /etc/xdg in that. But nah, stuff 'em.