[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

test/unit/specs/image-preview.spec.js

@@ -21,7 +21,12 @@ describe('ImagePreview', () => {
     ImagePreview(images);
     Vue.nextTick(() => {
       expect(document.querySelectorAll('.van-image-preview img').length).to.equal(3);
-      done();
+      ImagePreview(images.slice(0, 2));
+
+      Vue.nextTick(() => {
+        expect(document.querySelectorAll('.van-image-preview img').length).to.equal(2);
+        done();
+      });
     });
   });