backButtonImage support

lib/ios/RNNTopBarOptions.h

 @property (nonatomic, strong) NSNumber* largeTitle;
 @property (nonatomic, strong) NSString* testID;
 @property (nonatomic, strong) RNNTitleOptions* title;
+@property (nonatomic, strong) NSNumber* backButtonImage;
 
 @property (nonatomic, strong) NSString* componentName;
 @property (nonatomic, strong) NSString* backgroundComponentName;

lib/ios/RNNTopBarOptions.m

 		_navigationButtons = [[RNNNavigationButtons alloc] initWithViewController:(RNNRootViewController*)viewController];
 		[_navigationButtons applyLeftButtons:self.leftButtons rightButtons:self.rightButtons];
 	}
+	
+	UIImage *image = self.backButtonImage ? [RCTConvert UIImage:self.backButtonImage] : nil;
+	[[UINavigationBar appearance] setBackIndicatorImage:image];
+	[[UINavigationBar appearance] setBackIndicatorTransitionMaskImage:image];
 }
 
 -(void)storeOriginalTopBarImages:(UIViewController*)viewController {