Fix ControlW and ControlU keys in Completion mode.

After I hit <tab> and then try to press C-U or C-W it places a ^U or ^W in the line instead of deleting the line. This PR will fix that behavior. 
Make sure that you exit ``COMPLETE`` mode here:

```python
        # Quit autocomplete mode.
        if event.input_processor.input_mode == InputMode.COMPLETE:
            event.input_processor.pop_input_mode()
```