message: docs(changelog): 2.1.8
diff: # Changelog
  +### [v2.1.8](https://github.com/youzan/vant/tree/v2.1.8)
  +`2019-08-29`
  +
  +**Improvements**
  +
  +- Picker: add allow-html prop [\#4278](https://github.com/youzan/vant/pull/4278)
  +- NumberKeyboard: add focused prop [\#4279](https://github.com/youzan/vant/pull/4279)
  +- GoodsActionButton: add color prop [\#4255](https://github.com/youzan/vant/pull/4255)
  +- Button: color prop support linear-gradient [\#4252](https://github.com/youzan/vant/pull/4252)
  +
  +**Bug Fixes**
  +
  +- fix Circle stroke-width can't be greater than 60 [\#4256](https://github.com/youzan/vant/pull/4256)
  +- fix Picker should avoid click event when moving [\#4273](https://github.com/youzan/vant/pull/4273)
  +- fix Picker incorrect position when pause momentum [\#4277](https://github.com/youzan/vant/pull/4277)
  +
  +
  ### [v2.1.7](https://github.com/youzan/vant/tree/v2.1.7)
  `2019-08-26`
  