add backButtonImage to app style in order to custom the default back button arrow image with provided image (#1834)

docs/top-level-api.md

   appStyle: {
     orientation: 'portrait', // Sets a specific orientation to the entire app. Default: 'auto'. Supported values: 'auto', 'landscape', 'portrait'
     bottomTabBadgeTextColor: 'red', // Optional, change badge text color. Android only
-    bottomTabBadgeBackgroundColor: 'green' // Optional, change badge background color. Android only
+    bottomTabBadgeBackgroundColor: 'green', // Optional, change badge background color. Android only
+    backButtonImage: require('./pathToImage.png') // Change the back button default arrow image with provided image. iOS only
   },
   drawer: { // optional, add this if you want a side menu drawer in your app
     left: { // optional, define if you want a drawer from the left

ios/RCCManager.m

 #import "RCCViewController.h"
 #import <React/RCTBridge.h>
 #import <React/RCTRedBox.h>
+#import <React/RCTConvert.h>
 #import <Foundation/Foundation.h>
 
 @interface RCCManager() <RCTBridgeDelegate>
 {
   static RCCManager *sharedInstance = nil;
   static dispatch_once_t onceToken = 0;
-
+  
   dispatch_once(&onceToken,^{
     if (sharedInstance == nil)
     {
       sharedInstance = [[RCCManager alloc] init];
     }
   });
-
+  
   return sharedInstance;
 }
 
   {
     self.modulesRegistry = [@{} mutableCopy];
     
-//    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(onRNReload) name:RCTReloadNotification object:nil];
+    //    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(onRNReload) name:RCTReloadNotification object:nil];
   }
   return self;
 }
   {
     return;
   }
-
+  
   NSMutableDictionary *componentsDic = self.modulesRegistry[componentType];
   if (componentsDic == nil)
   {
     componentsDic = [@{} mutableCopy];
     self.modulesRegistry[componentType] = componentsDic;
   }
-
+  
   /*
-  TODO: we really want this error, but we need to unregister controllers when they dealloc
-  if (componentsDic[componentId])
-  {
-    [self.sharedBridge.redBox showErrorMessage:[NSString stringWithFormat:@"Controllers: controller with id %@ is already registered. Make sure all of the controller id's you use are unique.", componentId]];
-  }
-  */
-   
+   TODO: we really want this error, but we need to unregister controllers when they dealloc
+   if (componentsDic[componentId])
+   {
+   [self.sharedBridge.redBox showErrorMessage:[NSString stringWithFormat:@"Controllers: controller with id %@ is already registered. Make sure all of the controller id's you use are unique.", componentId]];
+   }
+   */
+  
   componentsDic[componentId] = controller;
 }
 
   {
     return nil;
   }
-
+  
   id component = nil;
-
+  
   NSMutableDictionary *componentsDic = self.modulesRegistry[componentType];
   if (componentsDic != nil)
   {
     component = componentsDic[componentId];
   }
-
+  
   return component;
 }
 
 -(void)initBridgeWithBundleURL:(NSURL *)bundleURL launchOptions:(NSDictionary *)launchOptions
 {
   if (self.sharedBridge) return;
-
+  
   self.bundleURL = bundleURL;
   self.sharedBridge = [[RCTBridge alloc] initWithDelegate:self launchOptions:launchOptions];
   
 -(void)setAppStyle:(NSDictionary*)appStyle
 {
   self.globalAppStyle = [NSDictionary dictionaryWithDictionary:appStyle];
+  [self applyAppStyle];
+  
+}
+
+-(void)applyAppStyle {
+  id backButtonImage = self.globalAppStyle[@"backButtonImage"];
+  UIImage *image = backButtonImage ? [RCTConvert UIImage:backButtonImage] : nil;
+  [[UINavigationBar appearance] setBackIndicatorImage:image];
+  [[UINavigationBar appearance] setBackIndicatorTransitionMaskImage:image];
 }