[Feature Request]input 组件的优化

### What problem does this feature solve?
input组件需要同时有选择和删除的功能，选择用append做，删除用icon图标做，希望能有个api能让icon在鼠标移入input的时候显示，并且同时用俩个的时候不会覆盖到一起
https://user-images.githubusercontent.com/23629370/35140447-3c31c0f4-fd32-11e7-86b2-7151a8732299.png
### What does the proposed API look like?
能让icon在鼠标移入input的时候显示，并且同时用俩个的时候不会覆盖到一起，把icon图标显示位置移到 slot="append" 前面最好了

Start by locating the existing input component and reading how its append slot and delete icon are currently handled. Verify the proposed API and hover behavior against the attached example. Done means selection and deletion can be enabled together, the icons do not overlap, and the icon appears on input hover.

Written by the indexing model from the issue text.