add disabledButtonColor (#1842)

* add disabledButtonColor in order to set navigation button disabled color

* add disabledButtonColor to the docs

docs/styling-the-navigator.md

   navBarSubtitleFontFamily: 'font-name', // subtitle font
   screenBackgroundColor: 'white', // Default screen color, visible before the actual react view is rendered
   orientation: 'portrait' // Sets a specific orientation to a modal and all screens pushed to it. Default: 'auto'. Supported values: 'auto', 'landscape', 'portrait'
+  disabledButtonColor: '#ff0000' // chnaged the navigation bar button text color when disabled.
   
   // iOS only
   statusBarTextColorSchemeSingleScreen: 'light', // same as statusBarTextColorScheme but does NOT remember across pushes

ios/RCCNavigationController.m

       [barButtonItem setImage:[barButtonItem.image imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal]];
     }
     
+    if ([viewController isKindOfClass:[RCCViewController class]]) {
+      RCCViewController *rccViewController = ((RCCViewController*)viewController);
+      NSDictionary *navigatorStyle = rccViewController.navigatorStyle;
+      id disabledButtonColor = navigatorStyle[@"disabledButtonColor"];
+      if (disabledButtonColor) {
+        UIColor *color = [RCTConvert UIColor:disabledButtonColor];
+        [barButtonItem setTitleTextAttributes:@{NSForegroundColorAttributeName : color} forState:UIControlStateDisabled];
+      }
+    }
+    
     NSString *testID = button[@"testID"];
     if (testID)
     {