message: Add aria-describedby and id for popover component and directive
diff: @@ -7,6 +7,7 @@ import { isBs3 } from 'ngx-bootstrap/utils';
  changeDetection: ChangeDetectionStrategy.OnPush,
  // tslint:disable-next-line
  host: {
  + '[attr.id]': 'popoverId',
  '[class]':
  '"popover in popover-" + placement + " " + "bs-popover-" + placement + " " + placement + " " + containerClass',
  '[class.show]': '!isBs3',
  @@ -42,6 +43,7 @@ export class PopoverContainerComponent {
  @Input() placement: string;
  @Input() title: string;
  containerClass: string;
  + popoverId: string;
  get isBs3(): boolean {
  return isBs3();
  