NbMenuItem to provide ability to have notification bubble

### Issue type

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

### Issue description

**Current behavior:**

I will start by saying, I absolutely love the simplicity and ease of use of NbMenuItem. I am only requesting an additional feature to be added.

**Expected behavior:**

I am looking to use this template for our custom built Help Desk application. We have different pages for actions like Approvals and Assignments. I would like to know if there is currently a way to show a notification badge next to the menu item to show the user that they need to visit that page. For example, if the logged in user had 3 approvals waiting their approval, there would be a 3 next to 'My Approvals'.

I tried to upload a photo, but it is not letting me. I can email the example, if needed.

**Related code:**

```
export declare class NbMenuItem {
/**
* Item Title
* @type {string}
*/
title: string;
/**
* Item relative link (for routerLink)
* @type {string}
*/
link?: string;
/**
* Item URL (absolute)
* @type {string}
*/
url?: string;
/**
* Icon class name
* @type {string}
*/
icon?: string;
/**
* Expanded by defaul
* @type {boolean}
*/
expanded?: boolean;
/**
* Children items
* @type {List}
*/
children?: NbMenuItem[];
/**
* Children items height
* @type {number}
*/
subMenuHeight?: number;
/**
* HTML Link target
* @type {string}
*/
target?: string;
/**
* Hidden Item
* @type {boolean}
*/
hidden?: boolean;
/**
* Item is selected when partly or fully equal to the current url
* @type {string}
*/
pathMatch?: string;
/**
* Where this is a home item
* @type {boolean}
*/
home?: boolean;
/**
* Whether the item is just a group (non-clickable)
* @type {boolean}
*/
group?: boolean;
/** Map of query parameters
*@type {Params}
*/
queryParams?: Params;
parent?: NbMenuItem;
selected?: boolean;
data?: any;
fragment?: string;
}
```