[new feature] Image: add loading-placeholder prop (#3893)

src/image/test/__snapshots__/index.spec.js.snap

@@ -24,3 +24,5 @@ exports[`load event 2`] = `
       <!----></i></div>
 </div>
 `;
+
+exports[`loading-placeholder prop 1`] = `<div class="van-image"><img class="van-image__img"></div>`;

src/image/test/index.spec.js

@@ -101,3 +101,13 @@ test('lazy-load error event', done => {
     }
   });
 });
+
+test('loading-placeholder prop', () => {
+  const wrapper = mount(Image, {
+    propsData: {
+      loadingPlaceholder: false
+    }
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});