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.
As, yes, I didn't took into account an empty config, I stayed in the optics that we were checking for the `None` set before. Thanks for clarifying.