Adding preferredContentSize (#2529)

* added the ability to change the backing controller's preferredContentSize through props.

* added some documentation outlining preferredContentSize and modalPresentationStyle properties

docs/styling-the-navigator.md

   topBarShadowOpacity: 0.5, // Sets shadow opacity on the navbar, Works only when topBarElevationShadowEnabled: true
   topBarShadowOffset: 12, // Sets shadow offset on the navbar, Works only when topBarElevationShadowEnabled: true
   topBarShadowRadius: 3 // Sets shadow radius on the navbar, Works only when topBarElevationShadowEnabled: true
+
+  preferredContentSize: { width: 500, height: 500 } // Sets the preferred size for the view controller’s view.
+  modalPresentationStyle: 'formSheet' // Sets the presentation style for modally presented view controllers. Supported styles are: 'formSheet', 'pageSheet', 'overFullScreen' and 'fullScreen'. 
   
   // Android only
   navigationBarColor: '#000000', // change the background color of the bottom native navigation bar.

ios/RCCViewController.m

   } else {
     self._statusBarHidden = NO;
   }
+  
+  NSDictionary *preferredContentSize = self.navigatorStyle[@"preferredContentSize"];
+  if (preferredContentSize) {
+    NSNumber *width = preferredContentSize[@"width"];
+    NSNumber *height = preferredContentSize[@"height"];
+    if (width && height) {
+      self.preferredContentSize = CGSizeMake([width floatValue], [height floatValue]);
+    }
+  }
 }
 
 - (BOOL)hidesBottomBarWhenPushed