Use NbChatComponent without affecting existing styles

### Issue type

**I'm submitting a ...**

* [] bug report
* [x] feature request

### Issue description

**Current behavior:**
I'm trying to render a `NbChatComponent`.

I've followed the manual installation guide as suggested [here](https://akveo.github.io/nebular/docs/guides/install-nebular#manually) and used the `NbChatComponent` as shown in [your example code](https://akveo.github.io/nebular/docs/components/chat-ui/overview#nbchatcomponent).

Later I realized, Im forced to add global level styles just to include that one component which I need on just one of my pages. My application obviously has its own styling from bootstrap, custom files etc and don't want to override those. The moment I include the global styles from Nebular, the existing styles are completely changed (links on hover changes its color, text fonts and font-weights have changed etc).

So, could you please suggest - how I can use **just** the `NbChatComponent` and its styling **by not affecting (overriding) my existing custom styles**.

**Expected behavior:**
I expected a component based architecture .ie. I just use the component and its style without having to touch the styles of elements outside the component scope.

**Steps to reproduce:**
Please find a Stackblitz URL below.

**Related code:**
https://stackblitz.com/edit/github-vphcit

### Other information:

**npm, node, OS, Browser**
```
Node, npm: `v10.4.0` and `6.13.0`
OS: macOS (Mojave - 10.14.6)
Browser: Chrome
```

**Angular, Nebular**
```
Angular: 8
Nebular: 4.6.0
```