Determines the text that will be displayed in the header's title.

format string/View Option Hash, default:

    month: 'MMMM YYYY', // September 2009
    week: "MMM D YYYY", // Sep 13 2009
    day: 'MMMM D YYYY'  // September 8 2009

The default value will change based on the current lang.

A single format string will set the title format for all views. A View Option Hash may be provided to target specific views (this is what the default does).