pass imageInsets for tab icon [iOS] (#1138)

Pass iconInsets for tab icon [iOS]

docs/top-level-api.md

       screen: 'example.FirstTabScreen', // unique ID registered with Navigation.registerScreen
       icon: require('../img/one.png'), // local image asset for the tab icon unselected state (optional on iOS)
       selectedIcon: require('../img/one_selected.png'), // local image asset for the tab icon selected state (optional, iOS only. On Android, Use `tabBarSelectedButtonColor` instead)
+      iconInsets: { // add this to change icon position (optional, iOS only).
+        top: 6, // optional, default is 0.
+        left: 0, // optional, default is 0.
+        bottom: -6, // optional, default is 0.
+        right: 0 // optional, default is 0.
+      },
       title: 'Screen One', // title of the screen as appears in the nav bar (optional)
       navigatorStyle: {}, // override the navigator style for the tab screen, see "Styling the navigator" below (optional),
       navigatorButtons: {} // override the nav buttons for the tab screen, see "Adding buttons to the navigator" below (optional)
 
 ```js
 Navigation.registerScreen('example.AdvancedScreen', () => AdvancedScreen);
-```
+```

ios/RCCTabBarController.m

     viewController.tabBarItem.accessibilityIdentifier = tabItemLayout[@"props"][@"testID"];
     viewController.tabBarItem.selectedImage = iconImageSelected;
     
+    id imageInsets = tabItemLayout[@"props"][@"iconInsets"];
+    if (imageInsets && imageInsets != (id)[NSNull null])
+    {
+      id topInset = imageInsets[@"top"];
+      id leftInset = imageInsets[@"left"];
+      id bottomInset = imageInsets[@"bottom"];
+      id rightInset = imageInsets[@"right"];
+      
+      CGFloat top = topInset != (id)[NSNull null] ? [RCTConvert CGFloat:topInset] : 0;
+      CGFloat left = topInset != (id)[NSNull null] ? [RCTConvert CGFloat:leftInset] : 0;
+      CGFloat bottom = topInset != (id)[NSNull null] ? [RCTConvert CGFloat:bottomInset] : 0;
+      CGFloat right = topInset != (id)[NSNull null] ? [RCTConvert CGFloat:rightInset] : 0;
+      
+      viewController.tabBarItem.imageInsets = UIEdgeInsetsMake(top, left, bottom, right);
+    }
+    
     NSMutableDictionary *unselectedAttributes = [RCTHelpers textAttributesFromDictionary:tabsStyle withPrefix:@"tabBarText" baseFont:[UIFont systemFontOfSize:10]];
     if (!unselectedAttributes[NSForegroundColorAttributeName] && buttonColor) {
       unselectedAttributes[NSForegroundColorAttributeName] = buttonColor;