Adds onMessage callback for non default handlers.

ios/RNCWebView.m

       [event addEntriesFromDictionary: @{@"data": message.body}];
       _onMessage(event);
     }
+  } else {
+    if (_onMessage) {
+      NSMutableDictionary<NSString *, id> *event = [self baseEvent];
+      [event addEntriesFromDictionary: @{@"data":    message.body}];
+      [event addEntriesFromDictionary: @{@"handler": message.name}];
+      _onMessage(event);
+    }
   }
 }