Hardcoded input box label

[Lasloruhberg]

The filechooser input box is hardcoded to display 'output filename' in

https://github.com/crahan/ipyfilechooser/blob/e4f82f4946b0db718a10b515669be9d4f731e00f/ipyfilechooser/filechooser.py#L66

if no filename is given. It would be nice to be able to modify this to e.g. "Input filename".