Clear the value of internal input if cleared value passed as prop

### I'm Submitting a ...

```
[ ] Bug report
[x] Feature request
[ ] Support request
```

### Expected Results

Without creating a wrapper, it should be possible to clear the value of the `Datetime`'s component input. By having a cleared value, the `Datetime` component should clear the input's value too.

### Actual Results

The `Datetime` component's value is cleared but leaves the input's value uncleared.

### Minimal Reproduction of the Problem

1. Open https://codesandbox.io/s/clear-react-datetime-4zur7
2. Click on the input
3. Select a date via the calendar
4. Press the `clear` button to clear the value passed to `Datetime`