feat(datepicker): add helpers for styling

- Add helper variables available for custom templates to hook into for styling

Closes #5580

Author: wesleycho

src/datepicker/datepicker.js

@@ -310,12 +310,17 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
   this.createDateObject = function(date, format) {
     var model = ngModelCtrl.$viewValue ? new Date(ngModelCtrl.$viewValue) : null;
     model = dateParser.fromTimezone(model, ngModelOptions.timezone);
+    var today = new Date();
+    today = dateParser.fromTimezone(today, ngModelOptions.timezone);
+    var time = this.compare(date, today);
     var dt = {
       date: date,
       label: dateParser.filter(date, format),
       selected: model && this.compare(date, model) === 0,
       disabled: this.isDisabled(date),
-      current: this.compare(date, new Date()) === 0,
+      past: time < 0,
+      current: time === 0,
+      future: time > 0,
       customClass: this.customClass(date) || null
     };