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.
`[::-1]` is a simple thing to learn, but for someone not familiar with the slicing syntax, it's probably not at all obvious what's going on, and it's not easy to search for.