Chat UI - textarea instead of input / disable send on enter

### Issue type

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

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
- Text input will not allow line breaks
- Enter button sends message

**Expected behavior:**
- Input will be a textarea element
- Enter button will insert a line break

**Steps to reproduce:**
Include any basic ChatUI element

### Other information:

**npm, node, OS, Browser**

**Angular, Nebular**
```
Angular CLI: 8.3.20
Nebular: 4.5.0
```