Add auto-scroll option to sky city-list example

sky/examples/city-list/city-list.sky

Index: sky/examples/city-list/city-list.sky
diff --git a/sky/examples/city-list/city-list.sky b/sky/examples/city-list/city-list.sky
index c1ecc67aa844fea88b3e137e7b89d7521e73f5a6..f89cad77fc524a417f5c8537cc54e44b29d14dc8 100644
--- a/sky/examples/city-list/city-list.sky
+++ b/sky/examples/city-list/city-list.sky
@@ -599,6 +599,11 @@ SkyElement({
           return;
 
         this.updateView(this.loader.getItems(), true);
+      },
+
+      scrollBy: function(amount) {
+        this.scrollerElement.scrollTop += amount;
+        this.handleScroll({ target: this.scrollerElement });
       }
     });