Add onPressHeader prop on Calendar

[Altimar21]

Hello,

This is an update for https://github.com/wix/react-native-calendars/pull/482. Add prop onPressHeader on Header component.

This is my first contribution, if I need to correct something say me,

Thanks,

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[goxr3plus]

...

[kiranojhanp]

.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.