new sandbox strategy

In this commit we switched the sandbox strategy to use iframe's
sandbox attribute. Indeed we do not need delegate event anymore,
but need to add some styles into the iframe.
The details were documented in the sandbox part of internal design.

README.md

@@ -18,7 +18,6 @@ rrweb 主要由 3 部分组成：
   - 处理跨域请求错误
   - 转移至 web worker 中执行
   - 实现传输数据压缩
-  - 移除 inline script
   - 验证移动端录制效果
 - rrweb-player
   - 改进播放器 UI 样式
@@ -37,6 +36,7 @@ rrweb 主要由 3 部分组成：
 - [序列化](./docs/serialization.md)
 - [增量快照](./docs/observer.md)
 - [回放](./docs/replay.md)
+- [沙盒](./docs/sandbox.md)
 
 ## Contribute Guide