Improve inputDate.js to allow the management of selectable days

Allow the management of selectable days through function "isSelectable" in dateinput parameters.

Example, to prevent selection of sundays  :
$(".input-date").dateinput({ 
    isSelectable: function (date) {
        if(date.getDay() == 0) return false;
        return true;
    }
});

Selection of days can be improved with more complex functions.