Changing 'hh' and 'h' format parameters to convert to '12' rather than '0' at midnight

The native datetime format option for 12-hour time (%I) converts 12-midnight to '12' rather than '0'

```python
datetime(2013, 1, 1).strftime('%I:%M %p') -> '12:00 AM'
```

existing Arrow code


```python
arrow.get(2013, 1, 1).format('hh:mm A') -> '00:00 AM'
```