feat(calendar): "get date" can return formatted date

[dreaming-augustin]

$('.ui.calendar').calendar('get date');
$('.ui.calendar').calendar('get focusDate');
$('.ui.calendar').calendar('get startDate');
$('.ui.calendar').calendar('get endDate');

could so far only return a Javascript Date object.

Now, given a format argument, they return a formatted date string:

$('.ui.calendar').calendar('get date',      "YYYY-MM-DD");
$('.ui.calendar').calendar('get focusDate', "YYYY-MM-DD");
$('.ui.calendar').calendar('get startDate', "YYYY-MM-DD");
$('.ui.calendar').calendar('get endDate',   "YYYY-MM-DD");

The format string is passed to the existing formatter.

This is backward compatible as the default behaviour does not change.

It makes it convenient for developers to get exactly what they need when they need it.

[lubber-de]

Please see my comments / suggestions.
The new code is now somewhat duplicated four times (using a different metadata value) we should even think of putting that in a unique function

[dreaming-augustin]

1. Default function parameter values are not supported in IE11 (which FUI 2.9.x still supports)

Ok. I am not fluent in JavaScript. I think more in term of C++, so I couldn't imagine parameters that were undefined.

[dreaming-augustin]

The new code is now somewhat duplicated four times (using a different metadata value) we should even think of putting that in a unique function

Done. You may suggest another function name...

[lubber-de]

Thanks, but cleaner! LGTM
Please take a short look at your docs PR where i was already commenting fomantic/Fomantic-UI-Docs#524

[lubber-de]

Docs added by fomantic/Fomantic-UI-Docs#524

[dreaming-augustin]

Thank you for your reviews.
I apologize for dumping so many PR just before Christmas!
I hope you had/are having great holidays.