Bug: overriden `id` of ng-option component

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description
I think `NbOptionComponent` is not working as expected

**Current behavior:**
`NbOptionComponent` [is setting an `id`](https://github.com/akveo/nebular/blob/33f3a08dcb6779014669b7a04e20fe4f7434ad09/src/framework/theme/components/option/option.component.ts#L128) with no option to set it like
```html

```

If I inspect that element:
`id = nb-option-$`

**Expected behavior:**

```html

```
__SHOULD__ set `id = my-id`

and

```html

```
__SHOULD__ set `id = nb-option-$`

**Steps to reproduce:**
Using `nb-option` component anywhere

**Related code:**
```html

```

### Other information:

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