修复表单组件样式和单元测试用例 (#7)

* fix: loading small style, search style and dialog style

* fix: scroll to top

* fix mobile scroll

* fix scroll to top

* 文档细节优化

* unit test

* dialog and image-preview unit test

* fix form component style

* fix radio and checkbox style

* fix search component style

packages/vant-css/src/checkbox.css

@@ -15,6 +15,7 @@
       height: 22px;
       margin-right: 15px;
       float: left;
+      -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
     }
 
     @e control {
@@ -31,6 +32,7 @@
       line-height: 22px;
       margin-left: 37px;
       display: block;
+      -webkit-tap-highlight-color: rgba(0, 0, 0, 0);      
     }
 
     .van-icon {