Custom template support is not available for nbSelect

### Issue type

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

* [ ] bug report
* [x] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**

Custom template support is not available for nbSelect

**Expected behavior:**

Need to support custom template options with nbSelect for Rich UI representation.

**Steps to reproduce:**

Feature Request

**Angular, Nebular**

ng cli 8.0 and Nebular 4.6.0
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