Ipyfilechooser

Updates:
- Option added to specify a file filter (andriykorchak) https://github.com/crahan/ipyfilechooser/pull/24
- Add support for `ValueWidget` and `get_interact_value()` https://github.com/crahan/ipyfilechooser/issues/28
- Updated sample notebook with filter example
- Updated Development Status to Production/Stable

Updates:
- Option added to only display folders (andriykorchak) https://github.com/crahan/ipyfilechooser/pull/23
- Updated sample notebook and documentation

Updates:
- Option added to display folder icons (ptooley) https://github.com/crahan/ipyfilechooser/pull/16
- Updated sample notebook and documentation

Updates:
- Option added to add `self` as an argument to the callback function (ptooley) https://github.com/crahan/ipyfilechooser/pull/14

Updates:
- Return `None` if file is not selected (danjjl)

Updates:
- Option to register a callback function (`register_callback(function_name)`)