feat(CellGroup): add title slot (#4227)

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

@@ -1,5 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`CellGroup title slot 1`] = `
+<div>
+  <div class="van-cell-group__title">CustomTitle</div>
+  <div class="van-cell-group van-hairline--top-bottom"></div>
+</div>
+`;
+
 exports[`arrow direction 1`] = `
 <div class="van-cell van-cell--clickable"><i class="van-icon van-icon-arrow-down van-cell__right-icon">
     <!----></i></div>

src/cell/test/index.spec.js

@@ -1,4 +1,5 @@
 import Cell from '..';
+import CellGroup from '../../cell-group';
 import { mount } from '../../../test/utils';
 
 test('click event', () => {
@@ -56,3 +57,13 @@ test('title-style prop', () => {
 
   expect(wrapper).toMatchSnapshot();
 });
+
+test('CellGroup title slot', () => {
+  const wrapper = mount(CellGroup, {
+    scopedSlots: {
+      title: () => 'CustomTitle'
+    }
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});