feat(datepicker): add support for placement "auto"

Closes #1257
Closes #1820

src/datepicker/datepicker-input.ts

@@ -22,7 +22,7 @@ import {NgbDatepicker, NgbDatepickerNavigateEvent} from './datepicker';
 import {DayTemplateContext} from './datepicker-day-template-context';
 import {NgbDateParserFormatter} from './ngb-date-parser-formatter';
 
-import {positionElements} from '../util/positioning';
+import {positionElements, PlacementArray} from '../util/positioning';
 import {NgbDateStruct} from './ngb-date-struct';
 import {NgbCalendar} from './ngb-calendar';
 import {NgbDatepickerService} from './datepicker-service';
@@ -103,12 +103,13 @@ export class NgbInputDatepicker implements OnChanges,
  */
  @Input() outsideDays: 'visible' | 'collapsed' | 'hidden';
 
-
  /**
- * Placement of a datepicker popup. Accepts: "top", "bottom", "left", "right", "bottom-left",
- * "bottom-right" etc.
- */
- @Input() placement = 'bottom-left';
+ * Placement of a datepicker popup accepts:
+ * "top", "top-left", "top-right", "bottom", "bottom-left", "bottom-right",
+ * "left", "left-top", "left-bottom", "right", "right-top", "right-bottom"
+ * and array of above values.
+ */
+ @Input() placement: PlacementArray = 'bottom-left';
 
  /**
  * Whether to display days of the week