Ability to set a screenBackgroundColor on iOS (#437)

* To avoid white flash while component loads

ios/RCCViewController.m

 
 -(void)setStyleOnAppearForViewController:(UIViewController*)viewController
 {
+    NSString *screenBackgroundColor = self.navigatorStyle[@"screenBackgroundColor"];
+    if (screenBackgroundColor)
+    {
+        UIColor *color = screenBackgroundColor != (id)[NSNull null] ? [RCTConvert UIColor:screenBackgroundColor] : nil;
+        self.view.backgroundColor = color;
+    }
+ 
     NSString *navBarBackgroundColor = self.navigatorStyle[@"navBarBackgroundColor"];
     if (navBarBackgroundColor)
     {