message: [changelog] 0.12.4
diff: ## Changelog
  +### [0.12.4](https://github.com/youzan/vant/tree/v0.12.4)
  +`2018-01-18`
  +
  +**Improvements**
  +
  +* Picker: add props to custom button text [\#548](https://github.com/youzan/vant/pull/548) [@chenjiahan](https://github.com/chenjiahan)
  +* Toast: add setDefaultOptions method [\#541](https://github.com/youzan/vant/pull/541) [@chenjiahan](https://github.com/chenjiahan)
  +* Dialog: add setDefaultOptions method [\#539](https://github.com/youzan/vant/pull/539) [@chenjiahan](https://github.com/chenjiahan)
  +
  +**Bug Fixes**
  +
  +* Stepper: not fire event on changing the value prop [\#546](https://github.com/youzan/vant/pull/546) [@chuangbo](https://github.com/chuangbo)
  +* Picker: fix misspelling of visibleItemCount prop [\#549](https://github.com/youzan/vant/pull/549) [@chenjiahan](https://github.com/chenjiahan)
  +
  ### [0.12.3](https://github.com/youzan/vant/tree/v0.12.3)
  `2018-01-12`
  