message: [changelog] 0.12.8
diff: ## Changelog
  +
  +### [0.12.8](https://github.com/youzan/vant/tree/v0.12.8)
  +`2018-02-07`
  +
  +**Improvements**
  +* add Circle component [\#608](https://github.com/youzan/vant/pull/608) [@chenjiahan](https://github.com/chenjiahan)
  +* Tab: add title slot [\#603](https://github.com/youzan/vant/pull/603) [@chenjiahan](https://github.com/chenjiahan)
  +* Toast: adjust background color [\#601](https://github.com/youzan/vant/pull/601) [@chenjiahan](https://github.com/chenjiahan)
  +* Popup: support getContaienr [\#611](https://github.com/youzan/vant/pull/611) [@chenjiahan](https://github.com/chenjiahan)
  +* Sku: support imgage upload [\#612](https://github.com/youzan/vant/pull/612) [@w91](https://github.com/w91)
  +* Sku: support custom stepper [\#600](https://github.com/youzan/vant/pull/600) [@w91](https://github.com/w91)
  +
  +**Bug Fixes**
  +* fix Picker style error in some andriod devices [\#609](https://github.com/youzan/vant/pull/609) [@chenjiahan](https://github.com/chenjiahan)
  +* fix TreeSelect arrow position error [\#605](https://github.com/youzan/vant/pull/605) [@chenjiahan](https://github.com/chenjiahan)
  +
  +
  ### [0.12.7](https://github.com/youzan/vant/tree/v0.12.7)
  `2018-01-31`
  