Add StripeEvent.listening?

Often, I find it helpful in my retrievers to know whether `StripeEvent` has any registered listeners for a particular event type. This allows me to avoid a `Stripe::Event.retrieve` API call to Stripe if I am not even interested in processing the event. Currently, I do this in my retrievers:

```ruby
class StripeEventRetriever
  def call(params)
    return nil unless processing_events_of_type?(params[:type])
    # <snip>
  end

private
  def processing_events_of_type?(event_type)
    namespaced_type = StripeEvent.namespace.call(event_type)
    StripeEvent.backend.notifier.listening?(namespaced_type)
  end
end
```

Since `StripeEvent.namespace` and `StripeEvent.backend` are internal implementation details, I think it would be nice to provide a `StripeEvent.listening?` API which provides this functionality without exposing users of the gem to notification internals.