Changed backButton.hideTitle to backButton.showTitle

lib/ios/RNNBackButtonOptions.h

 @property (nonatomic, strong) NSNumber* visible;
 @property (nonatomic, strong) NSString* title;
 @property (nonatomic, strong) NSString* transition;
-@property (nonatomic, strong) NSNumber* hideTitle;
+@property (nonatomic, strong) NSNumber* showTitle;
 
 @end

lib/ios/RNNBackButtonOptions.m

 		viewController.navigationItem.hidesBackButton = ![self.visible boolValue];
 	}
 	
-	if (self.hideTitle) {
+	if (self.showTitle && ![self.showTitle boolValue]) {
 		self.title = @"";
 	}