Add dismissHandler to BLTNItemManager

Hi @alexisakers thanks for the great library!

Recently, I needed to know when a BLTNItemManager has dismissed and didn't want to implement a dismiss handler and every BLTNItem so I added a publicly accessible dismissHandler to the BLTNItemManager class. I would make a PR but since it's a small addition, thought Id just comment the code here.

```
@objc public final class BLTNItemManager: NSObject {

/// Executes custom function when BLTNItemManager is dismissed
public var dismissHandler: (() -> Void)?

}
```

```
@nonobjc func completeDismissal() {

// Calling BLTNManager's Main Dismiss Block (Set by User)
dismissHandler?()

}
```

I followed the logic in this S/O answer: https://stackoverflow.com/a/45619821/8074346