[new feature] ImagePreview support startPosition (#286)

* [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

packages/image-preview/index.js

@@ -12,12 +12,13 @@ const initInstance = () => {
   document.body.appendChild(instance.$el);
 };
 
-const ImagePreviewBox = images => {
+const ImagePreviewBox = (images, startPosition = 0) => {
   if (!instance) {
     initInstance();
   }
 
   instance.images = images;
+  instance.startPosition = startPosition;
   instance.value = true;
 };