weex: set append="tree" by default for <cell> component (#4326)

src/platforms/weex/compiler/modules/append.js

@@ -1,6 +1,10 @@
 /* @flow */
 
 function transformNode (el: ASTElement, options: CompilerOptions) {
+  if (el.tag === 'cell' && !el.attrsList.some(item => item.name === 'append')) {
+    el.attrsMap.append = 'tree'
+    el.attrsList.push({ name: 'append', value: 'tree' })
+  }
   if (el.attrsMap.append === 'tree') {
     el.appendAsTree = true
   }