[improvement] Button: add id prop (#650)

packages/button/README.md

@@ -57,6 +57,7 @@
 
 | 参数 | 说明 | 类型 | 默认值 |
 |-----------|-----------|-----------|-------------|
+| id | 标识符 | `String` | - |
 | type | 按钮类型，可选值为 `primary` `warning` `danger` | `String` | `default` |
 | size | 按钮尺寸，可选值为 `normal` `large` `small` `mini` | `String` | `normal` |
 | plain | 是否为朴素按钮 | `Boolean` | `false` |

packages/button/index.wxml

@@ -1,5 +1,6 @@
 <button
   class="custom-class van-button {{ classes }}"
+  id="{{ id }}"
   disabled="{{ disabled }}"
   hover-class="button-hover"
   open-type="{{ openType }}"

packages/mixins/button.ts

@@ -1,5 +1,6 @@
 export const button = Behavior({
   properties: {
+    id: String,
     loading: Boolean,
     openType: String,
     appParameter: String,