NbAutoComplete control - scroll support to arrow up/down keys

### Issue type

**I'm submitting a ...**

* [ ] bug report
* [ x] feature request

### Issue description

**Current behavior:**
Issue is related to the nbautocomplete https://akveo.github.io/nebular/docs/components/autocomplete/overview#nbautocompletedirective

When you scroll down using down arrow key, scroll will not go down and we can not see the items. Same behavior is for arrow up key.


**Expected behavior:**
Scroll should be possible using arrow keys as well.

**Steps to reproduce:**
1. Go to above **"Grouping"** example.
2. Try to scroll down with down arrow key. (not clicking on the scroll bar)