[new feature] Stepper add plus & minus event (#294)

* [bugfix] CouponList always show empty info

* [bugfix] add click feedback of buttons in components

* [Doc] add custom theme document

* [new feature] Notice bar support more props

* [bugfix] PullRefresh test cases

* [bugfix] unused NoticeBar style

* [bugfix] Swipe width calc error

* [Doc] english document of all action components

* [Doc] change document site path to /zanui/vant

* [Doc] fix

* [bugfix] uploader style error

* [bugfix] tabs document demo

* [new feature] Cell support vue-router target route

* [bugfix] add cell test cases

* update yarn.lock

* [bugfix] Tabbar cann't display info when use icon slot

* [Doc] update document title

* [bugfix] Dialog should reset button text when showed

* [new feature] CouponList add showCloseButton prop

* [new feature] Swipe add 'initialSwipe' prop

* [bugfix] NoticeBar text disappeared when page back

* [new feature] ImagePreview support startPosition

* fix: improve imagePreview test cases

* [bugfix] Steps style error when has more than 4 items

* [new feature] normalize size of all icons

* [new feature] Stepper add plus & minus event

* fix: yarn.lock

* [bugfix] addressEdit icon render failed

docs/examples-docs/en-US/stepper.md

@@ -77,3 +77,5 @@ export default {
 |-----------|-----------|-----------|
 | change | Triggered when value change | value: current value |
 | overlimit | Triggered when click disabled button | - |
+| plus | Triggered when click plus button | - |
+| minus | Triggered when click minus button | - |

docs/examples-docs/zh-CN/stepper.md

@@ -82,3 +82,5 @@ export default {
 |-----------|-----------|-----------|
 | change | 当绑定值变化时触发的事件 | 当前组件的值 |
 | overlimit | 点击不可用的按钮时触发 | - |
+| plus | 点击增加按钮时触发 | - |
+| minus | 点击减少按钮时触发 | - |