[Chat UI] message input "date" should allow for different date formatting

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**

I saw in https://github.com/akveo/nebular/blob/master/src/framework/theme/components/chat/chat-message-text.component.ts#L15 that the `date` input into `nb-chat-message` is always piped into `shortTime`. This is confusing if the messages span multiple days. Can something like `datePipeFormat: string` (https://angular.io/api/common/DatePipe#pre-defined-format-options) be added as an input as well?

**Expected behavior:**

The chat messages would display the date as well to prevent confusing over messages spanning multiple days.

**Steps to reproduce:**

Add messages to `nb-chat-message` where the dates span multiple days.

### Other information:

**npm, node, OS, Browser**
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```

```
node --version
v10.16.0

npm --version
6.9.0

OS: macOS Mojave
Browser: Chrome
```