Denying subscription to topic in wamp
Is there any way currently to issue a check/authorisation on the server side when a client is trying to subscribe to the topic, or the only way is to redeclare the `WampServer` and `TopicManager` to add the check to `TopicManager::onSubscribe()`?

Is there any reason to why the [app's method](https://github.com/cboden/Ratchet/blob/master/src/Ratchet/Wamp/TopicManager.php#L48) is called after adding the client or would it be possible to move that up, and if it throws an auth exception catch and don't add the client at all?

Or introduce a new function in the `WampServerInterface` as `onSubscribeReq` which should be defined in the app and ran first in `TopicManager::onSubscribe()` in a `try`?
