RTL support improvement

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
For now, we have only `NbLayoutDirectionService` which provides information about a state of our layout. So, when we using ChangeDetectionStrategy.OnPush in component and wanna use layout service we have to trigger change detection manually or use `async` pipe, for example.

**Expected behavior:**
I want to suggest building two directives: `nbIfRtl` and `nbIfLtr`. Both this directives have to support 'else' syntax like `ngIf`.
This directive will help us create cleaner code without manually subscriptions management in each component which uses `ChangeDetectionStrategy.OnPush` and `NbLayoutDirectionService`.