Add `locale.headerDateFormat` option

[stegano]

Add locale.headerDateFormat option

It can customize format the top of the calendar.

The headerDateFormat option has been added. Use this option to specify the date format at the top of calendar. Also, changed the name of the existing format option to dateFormat. This explains the function of more specific options. The existing format option is still compatible but is not recommended.

relates: #1501 #1524 #1785

[dwasyl]

This would be a great feature, hope it gets merged.

[stegano]

ping @dangrossman