autocomplete item extension

* add `target` attribute, as final input-value after selection
* add new test case for the feature
* update existing test cases to run ok
* add example for the feature

新搞一个分支, 添加了测试用例和demo, 重新PR.

你们的开发环境和开发方式跟我的比起来简直高大上啊, 平时还没这样玩过, 算是跟你们学习一把, 我还太菜了.