Icons Pack: Register a svg

### Issue type

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

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
Registering 3rd Party Icons with

```
constructor(private iconLibraries: NbIconLibraries) {
this.iconsLibrary.registerSvgPack('social-networks', {
'facebook': '',
'twitter': '',
// ...
});
```

**Expected behavior:**
Registering 3rd Party Icons from file.svg

```
constructor(private iconLibraries: NbIconLibraries) {
this.iconsLibrary.registerSvgPack('social-networks', {
'facebook': './assets/facebook.svg',
'twitter': './assets/twitter.svg',
// ...
});
```