nb-select: Select All, Unselect All, and type ahead search feature request

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**

Currently, the nb-select component does not support the following features:

1. Select All
2. Unselect All, and
3. Type ahead search features

**Expected behavior:**

It would be provide a great usability boost if the above 3 features are supported by the nb-select component.