Added %list_extensions magic and ipython_extensions setuptools entry_points
OrderedDict.items for PY3 compat
Widen glob to match top-level directories (in-lieu of pathlib PEP 428 support in Python 2.7)
Renamed to %lsext, updated/extended docstrings
Updated docstrings for ExtensionManager.print_extensions and ExtensionMagic.listext
Return an ExtensionDisplay object with a _repr_pretty_
Added ipython_exporters entrypoints for nbconvert
Reverted explicit exporter imports