Verify subscription for multiple/all products?

### Platform

- [ ] iOS

### In app purchase type

- [ ] Auto-Renewable Subscription

### Environment

- [ ] Sandbox
- [ ] Production

### Version
0.8.6

### Related issues

## Report

### Issue summary
From your readme you call:
```SwiftyStoreKit.verifyReceipt(...) {
...
SwiftyStoreKit.verifySubscription(... productId: ...}
...
}
```
What to do if we don't know this ```productId``` and have multiple subscriptions which may vary?

### What did you expect to happen
Get any valid subscription from the request result without of binding to concrete Id?

### What happened instead
The app don't have to know ```productId``` but the result contains info about this subscription. Why must I know ```productId``` to check if just **ANY** subscription from list is valid?