[new feature] Checkbox: support config the maximum amount of checked options (#631)

test/unit/components/checkbox.vue

@@ -1,5 +1,5 @@
 <template>
-  <van-checkbox-group v-model="result">
+  <van-checkbox-group v-model="result" :max="max">
     <van-checkbox v-for="(item, index) in list" :key="index" :name="item" :disabled="index === 2">复选框{{item}}</van-checkbox>
   </van-checkbox-group>
 </template>
@@ -22,7 +22,8 @@ export default {
         'c',
         'd'
       ],
-      result: ['a', 'b']
+      result: ['a', 'b'],
+      max: 0
     };
   }
 };