Add new i18n option 'headerFormat' to datepicker

[harawata]

Proposed changes

To customize datepicker header format.
Please see #5905

Screenshots (if appropriate) or codepen:

Usage example:

$(".datepicker").datepicker({
  i18n: {
    headerFormat: 'm/d ddd'
  }
});

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I couldn't find tests for datepicker.
Please let me know if I missed it.

To minimize the change, I modified 'formats' functions and 'toString'.
If I should take a different approach, give me some advice and I'll update the PR.