docs: 更新 README；修复文档的 playground 页面切换图标不生效的问题；

Author: mathink12

README.md

@@ -1,2 +1,7 @@
 # json-tree
-JSON tree view for Vue
+
+JSON tree view for Vue.
+
+[Demo | Docs](https://mathink12.github.io/json-tree/)
+
+![demo](./docs/imgs/json-tree.gif)

docs/.vuepress/components/JsonTreeDemo.vue

@@ -43,8 +43,11 @@
       :json-data="jsonData"
       :show-line="showLine"
       :indent="indent"
-      :expand-deep="deep"
-    />
+      :expand-deep="deep">
+      <template v-if="toggle !== 'default'" #toggle="{ collapse }">
+        <i :class="calcToggleClass(collapse)"></i>
+      </template>
+    </JsonTree>
   </div>
 </template>