add system-wide IPython config

Mainly for use in multi-user administrated environments

- config files in `/usr[/local]/share/ipython` will be loaded
- change priority config loading to load all found files in priority order
  instead of loading only the first found.
By doing it like this, you won't yield an empty config that was successfully loaded. I'm not sure if it's necessary to avoid that, but it's not equivalent to doing `yield loader.load_config()`, or even putting the yield in the else clause.