Add option to select/auto complete to accept custom values or introducing a new component called tag-box

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
I have a use case, I want the user selects 1 or multiple items from the drop down list, but also be able to add a new option by typing and using enter key.

So it's like auto complete + multiple + add custom or select + multiple + add custom or tag-box + add-custom

Please see attached gif

**Expected behavior:**

**Steps to reproduce:**

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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