Feature(typeahead): Move Close popup into selectResult function

[kotarCreative]

Feature description:

The options window is closed before the _selectResult function is called. This means the window will always close regardless of the selectItem event being overridden. This prevents adequately creating a load more use case for large lists or server-side paginated lists.

It would be nice to either have the _closePopup function called within the defaultPrevented check or have the option to disable the popup closing on select and manually trigger it instead.

Versions of Angular, ng-bootstrap and Bootstrap:

Angular:
4.3.6
ng-bootstrap:
1.0.0
Bootstrap:
4