NbDummyAuthStrategy missing option to set redirect path

### Issue type

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

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
NbDummyAuthStrategy has no option to set redirect path after "successful" login. Hardcoded value redirects to `/`.

https://github.com/akveo/nebular/blob/master/src/framework/auth/strategies/dummy/dummy-strategy.ts#L67
```
return new NbAuthResult(true, this.createSuccessResponse(data), '/', [], ['Successfully logged in.'], token);
```

**Expected behavior:**

redirect option could be set if needed, as example:

```
export class NbDummyAuthStrategyOptions extends NbAuthStrategyOptions {
token?: NbStrategyToken = {
class: NbAuthSimpleToken,
};
delay?: number = 1000;
alwaysFail?: boolean = false;
redirect?: string = '/';
}
```

**Related code:**
https://github.com/akveo/nebular/blob/master/src/framework/auth/strategies/dummy/dummy-strategy-options.ts
https://github.com/akveo/nebular/blob/master/src/framework/auth/strategies/dummy/dummy-strategy.ts