Add new `month` prop

This prop should render the calendar displaying the given month:

```jsx
<DayPicker month={ new Date(2018, 12} />
```

similar to `initialMonth`, but it will re-render the component when its value is set to another month.
[See discussion here](https://github.com/gpbl/react-day-picker/issues/169#issuecomment-282011725)