Allow function value for dateFormat and timeFormat

### I'm Submitting a ...
```
[ ] Bug report
[x] Feature request
[ ] Support request
```

### Expected Results
It would add to flexibility of the component if we could supply a function for dateFormat and timeFormat props to implement display formatting not supported by moment.

This function would take the selected value as argument and return a string to be displayed.

A real world example where a format not supported by moment would be needed is a custom week of year formatting used by my company which doesn't correspond to any supported standards or localizations.

### Actual Results
Currently the component only takes a valid moment.js format string.