introducing navbar shadow on iOS (#2339)

* adds shadow opacity, shadow color, shadow offset, shadow radius to navbar for ios

* mentions ios shadow in styling-the-navigator docs

docs/styling-the-navigator.md

   navBarComponentAlignment: 'center', // center/fill
   navBarCustomViewInitialProps: {}, // navBar custom component props
   navBarButtonColor: '#007aff', // Change color of nav bar buttons (eg. the back button) (remembered across pushes)
-
+  topBarElevationShadowEnabled: false, // (Android - default: true, iOS - default: false). Disables TopBar elevation shadow on Lolipop and above
   navBarHidden: false, // make the nav bar hidden
   navBarHideOnScroll: false, // make the nav bar hidden only after the user starts to scroll
   navBarTranslucent: false, // make the nav bar semi-translucent, works best with drawUnderNavBar:true
   navBarRightButtonFontSize: 17, // Change font size of right nav bar button
   navBarRightButtonColor: 'blue', // Change color of right nav bar button
   navBarRightButtonFontWeight: '600', // Change font weight of right nav bar button
+
+  topBarShadowColor: 'blue' // Sets shadow of the navbar, Works only when topBarElevationShadowEnabled: true
+  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
   
   // Android only
   navigationBarColor: '#000000', // change the background color of the bottom native navigation bar.
   navBarTitleTextCentered: true, // default: false. centers the title.
   navBarButtonFontFamily: 'sans-serif-thin', // Change the font family of textual buttons
-  topBarElevationShadowEnabled: false, // default: true. Disables TopBar elevation shadow on Lolipop and above
   statusBarColor: '#000000', // change the color of the status bar.
   drawUnderStatusBar: false, // default: false, will draw the screen underneath the statusbar. Useful togheter with statusBarColor: transparent
   collapsingToolBarImage: "http://lorempixel.com/400/200/", // Collapsing Toolbar image.

ios/RCCViewController.m

     viewController.navigationController.navigationBar.tintColor = nil;
   }
   
+  BOOL topBarElevationShadowEnabled = self.navigatorStyle[@"topBarElevationShadowEnabled"] != (id)[NSNull null] ? [RCTConvert CGFloat:self.navigatorStyle[@"topBarElevationShadowEnabled"]] : NO;
+
+  if (topBarElevationShadowEnabled) {
+    CGFloat shadowOpacity = self.navigatorStyle[@"topBarShadowOpacity"] != 0 ? [RCTConvert CGFloat:self.navigatorStyle[@"topBarShadowOpacity"]] : 0.2;
+    CGFloat shadowOffset = self.navigatorStyle[@"topBarShadowOffset"] != 0 ? [RCTConvert CGFloat:self.navigatorStyle[@"topBarShadowOffset"]] : 3.0;
+    CGFloat shadowRadius = self.navigatorStyle[@"topBarShadowRadius"] != 0 ? [RCTConvert CGFloat:self.navigatorStyle[@"topBarShadowRadius"]] : 2.0;
+    UIColor *shadowColor = self.navigatorStyle[@"topBarShadowColor"] != (id)[NSNull null] ? [RCTConvert UIColor:self.navigatorStyle[@"topBarShadowColor"]] : UIColor.blackColor;
+
+    viewController.navigationController.navigationBar.layer.shadowOpacity = shadowOpacity;
+    viewController.navigationController.navigationBar.layer.shadowColor = shadowColor.CGColor;
+    viewController.navigationController.navigationBar.layer.shadowOffset = CGSizeMake(0, shadowOffset);
+    viewController.navigationController.navigationBar.layer.shadowRadius = shadowRadius;
+  }
+
   BOOL viewControllerBasedStatusBar = false;
   
   NSObject *viewControllerBasedStatusBarAppearance = [[NSBundle mainBundle] infoDictionary][@"UIViewControllerBasedStatusBarAppearance"];