*ngIf in nebular menu

### Issue type

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

* [x ] bug report
* [ ] feature request

### Issue description

**Current behavior:**

Before nebular I just used to simple:
`

Admin Board

`
**Expected behavior:**

How can I use something like *ngIf for checking is user is logged in or his role is Admin/Mod etc? I got my own backend and I am using my onw services for provide backend data.

But in nebular i got:
`
`
**Steps to reproduce:**

for example:
` items: NbMenuItem[] = [
{
title: 'Admin Board',
icon: 'star-outline',
link: 'admin'
},
];`

```
insert short code snippets here
```

### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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