[new feature] Area add 'title' prop & optimzie Picker performance (#450)

packages/picker/index.vue

@@ -19,6 +19,7 @@
         :visibileColumnCount="visibileColumnCount"
         @change="onChange(index)"
       />
+      <div class="van-picker__frame van-hairline--top-bottom" :style="frameStyle" />
     </div>
   </div>
 </template>
@@ -41,7 +42,10 @@ export default create({
       type: String,
       default: 'text'
     },
-    itemHeight: Number,
+    itemHeight: {
+      type: Number,
+      default: 44
+    },
     showToolbar: Boolean,
     visibileColumnCount: Number,
     columns: {
@@ -67,6 +71,14 @@ export default create({
     }
   },
 
+  computed: {
+    frameStyle() {
+      return {
+        height: this.itemHeight + 'px'
+      };
+    }
+  },
+
   methods: {
     initColumns() {
       const columns = this.columns.map(deepClone);