put dateFormatters to options, so we can overwrite/extend them if needed

i wanted an agenda week with just two rows (morning and afternoon)

e.g.
```
$myContainer.fullCalendar({
    ....
    axisFormat: 'tl', //rows - time
    slotMinutes: 360,
    defaultView: 'agendaWeek',
    minTime: 6,
    maxTime: 18,
    dateFormatters: {
        tl  : function(d)   { return d.getHours() < 12 ? 'vormittag' : 'nachmittag' },
    },
    ...
});
```