message: [changelog] 1.6.9
diff: ## Changelog
  +## [v1.6.9](https://github.com/youzan/vant/tree/v1.6.9)
  +`2019-03-11`
  +
  +**Improvements**
  +
  +- Field: add size prop [\#2936](https://github.com/youzan/vant/pull/2936)
  +- Collapse: add size prop [\#2946](https://github.com/youzan/vant/pull/2946)
  +- Collapse: add border prop [\#2933](https://github.com/youzan/vant/pull/2933)
  +- CellGroup: add title prop [\#2928](https://github.com/youzan/vant/pull/2928)
  +- Steps: add active-icon prop [\#2934](https://github.com/youzan/vant/pull/2934)
  +- Picker: add default-index prop [\#2935](https://github.com/youzan/vant/pull/2935)
  +- Actionsheet: add index param for select event [\#2917](https://github.com/youzan/vant/pull/2917)
  +
  +**Bug Fixes**
  +
  +- fix Card origin-price slot not work [\#2927](https://github.com/youzan/vant/pull/2927)
  +- fix Swipe emit change event in correct time [\#2909](https://github.com/youzan/vant/pull/2909)
  +
  +
  ## [v1.6.8](https://github.com/youzan/vant/tree/v1.6.8)
  `2019-03-02`
  