nb-chat component is missing localization

### Issue type: Bug/feature

* [x] bug report
* [x] feature request

### Issue description

**Current behavior:**
No-messages info. message is hardcoded (**No messages yet.**) in component template and cannot be easly translated.
`

No messages yet.

Same for message input placeholder (**Type a message**) in nb-chat-form component
``

**Expected behavior:**
Add @input properties to nb-chat to provide translations into templates

**Steps to reproduce:**
just use component

### Other information:

**npm, node, OS, Browser**
```
Node: v10.11.0, npm: 6.1.0
OS: Windows (10)
Browser: Chrome/Firefox/

```

**Angular, Nebular**
```
"@nebular/auth": "2.0.0-rc.10",
"@nebular/security": "2.0.0-rc.10",
"@nebular/theme": "2.0.0-rc.10",
```