[Menu] Need ID Property for NbMenuItem

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
Currently the NbMenuItem class does not have an ID property or other property to uniquely identify a menu item. This is necessary to remove menus dynamically or change their properties.

**Expected behavior:**
To have an ID property in the NbMenuItem class.

**Steps to reproduce:**
N/A

**Related code:**
N/A

### Other information:

**npm, node, OS, Browser**
```
Node: v9.3.0
NPM: 6.0.0
OS: Windows 10
Browser: Chrome, Safari
```

**Angular, Nebular**
```
Angular: 5.1.0
Nebular: 2.0.0-rc.7
```