* Slider: add format-tooltip prop

* Docs: update slider.md

* Docs: update slider.md

https://github.com/ElemeFE/element/issues/3800 fixed

修改为```var(--index-normal)```

tags的层级是1000，导致超出了Dialog的层级。
是否默认z-index:1;就够用了？

* Defined the tooltip component

* Removed component alias

* Form: 如果需要验证的fields为空，调用验证时立刻返回callback
同时修复几个代码格式用来通过npm run dist

* fix travis

* fix travis

* fix travis

* Add support for httpRequest Promise

添加支持httpRequest Promise

* fixbug

* fixbug

linux系统FF浏览器border显示不全

Select component with remote search clears what you typed when the value for each el-option is an object

Update cascader.css so that it looks better in non-white background

Changelog: update for 1.2.7

* Table: improve performance for rendering tooltip, fixed #3478

* Table: add tooltip-effect property