feat(sticky): add sticky-on active class on sticky ele

src/components/sticky/sticky.vue

@@ -15,7 +15,9 @@
 <script type="text/ecmascript-6">
   import {
     getRect,
-    prefixStyle
+    prefixStyle,
+    addClass,
+    removeClass
   } from '../../common/helpers/dom'
 
   const COMPONENT_NAME = 'cube-sticky'
@@ -102,6 +104,13 @@
               this.fixedEleHeight = 0
             }
           }
+          const onClass = 'cube-sticky-ele-on'
+          if (oldEle) {
+            removeClass(oldEle.$el, onClass)
+          }
+          if (newEle) {
+            addClass(newEle.$el, onClass)
+          }
         })
 
         this.currentKey = currentKey