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

docs/demos/views/area.vue

@@ -9,7 +9,7 @@
     </demo-block>
 
     <demo-block :title="$t('title3')">
-      <van-area :areaList="$t('areaList')" :columnsNum="2" />
+      <van-area :areaList="$t('areaList')" :columnsNum="2" :title="$t('title')" />
     </demo-block>
   </demo-section>
 </template>
@@ -34,7 +34,6 @@ export default {
 
   data() {
     return {
-      areaList: AreaList,
       value: '110101'
     };
   }

docs/markdown/en-US/area.md

@@ -32,7 +32,7 @@ To have a selected value，simply pass the `code` of target area to `value` prop
 Set `columnsNum` with 2, you'll have a 2 level picker.
 
 ```html
-<van-area :areaList="areaList" :columnsNum="2" />
+<van-area :areaList="areaList" :columnsNum="2" title="Title" />
 ```
 
 
@@ -41,6 +41,7 @@ Set `columnsNum` with 2, you'll have a 2 level picker.
 | Attribute | Description | Type | Default | Accepted Values |
 |-----------|-----------|-----------|-------------|-------------|
 | value | the `code` of selected area | `String` | - | - |
+| title | Toolbar title | `String` | `''` | - |
 | areaList | an object contains these properties: `province_list`, `city_list` and `county_list`  | `Object` | - | - |
 | columnsNum | level of picker | `String`,`Number` | 3 | - |
 

docs/markdown/zh-CN/area.md

@@ -33,7 +33,7 @@ Vue.use(Area);
 可以通过`columnsNum`属性配置省市县显示的列数，默认情况下会显示省市县，当你设置为`2`，则只会显示省市选择。
 
 ```html
-<van-area :areaList="areaList" :columnsNum="2" />
+<van-area :areaList="areaList" :columnsNum="2" title="标题" />
 ```
 
 ### API
@@ -41,6 +41,7 @@ Vue.use(Area);
 | 参数 | 说明 | 类型 | 默认值 | 可选值 |
 |-----------|-----------|-----------|-------------|-------------|
 | value | 当前选中的省市区`code` | `String` | - | - |
+| title | 顶部栏标题 | `String` | `''` | - |
 | areaList | 省市县数据，必须与`province_list`、`city_list`和`county_list`为key | `Object` | - | - |
 | columnsNum | 省市县显示列数，3-省市县，2-省市，1-省 | `String`,`Number` | 3 | - |