hideBackButtonTitle support

lib/ios/RNNTopBarOptions.h

 @property (nonatomic, strong) NSNumber* backButtonImage;
 @property (nonatomic, strong) NSNumber* backButtonHidden;
 @property (nonatomic, strong) NSString* backButtonTitle;
+@property (nonatomic, strong) NSNumber* hideBackButtonTitle;
 
 @property (nonatomic, strong) NSString* componentName;
 @property (nonatomic, strong) NSString* backgroundComponentName;

lib/ios/RNNTopBarOptions.m

 	[[UINavigationBar appearance] setBackIndicatorImage:image];
 	[[UINavigationBar appearance] setBackIndicatorTransitionMaskImage:image];
 	
+	if (self.hideBackButtonTitle) {
+		self.backButtonTitle = @"";
+	}
+	
 	if (self.backButtonTitle) {
 		UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithTitle:self.backButtonTitle
 																	 style:UIBarButtonItemStylePlain