Added delegate method to set visibility of cluster annotation's subtitle

I'm currently working on a project where I needed the ability to hide the subtitle of cluster annotations and only show a title with the format "%d shops". So I implemented an optional delegate method:

```objective-c
- (BOOL)shouldShowSubtitleForClusterAnnotationsInMapView:(ADClusterMapView *)mapView; // default: YES
```

It's called from the ADClusterMapView just before initializing the rootMapCluster.

I think that the ability could be helpful for others too.