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.
Real question, is there an advantage/inconvenient to `reversed` vs `[::-1]` ? I supposed that slicing make a copy but otherwise I don't see any advantages.