[improvement] Cell: 增加默认白色背景色 (#270)

* 修复stepper文档错误

* 采用极大数字来代替 infinity

* 增加 cell 背景色

* 增加外部样式类

example/pages/cell/index.json

@@ -3,6 +3,7 @@
   "usingComponents": {
     "zan-cell": "../../dist/cell/index",
     "zan-icon": "../../dist/icon/index",
+    "zan-badge": "../../dist/badge/index",
     "zan-panel": "../../dist/panel/index",
     "zan-cell-group": "../../dist/cell-group/index",
     "doc-page": "../../components/doc-page/index"

packages/cell-group/index.pcss

@@ -0,0 +1,3 @@
+.cell-group {
+  background-color: #fff;
+}

packages/cell/index.js

@@ -4,6 +4,7 @@ const warn = (msg, getValue) => {
 };
 
 Component({
+  externalClasses: ['cell-class'],
   options: {
     multipleSlots: true
   },

packages/cell/index.pcss

@@ -6,6 +6,7 @@
   display: flex;
   align-items: center;
   line-height: 1.4;
+  background-color: #fff;
   font-size: 14px;
 }
 

packages/cell/index.wxml

@@ -1,6 +1,6 @@
 <view 
   catchtap="cellTap"
-  class="zan-cell {{ isLastCell ? 'last-cell' : '' }} {{ isLink ? 'zan-cell--access' : '' }}">
+  class="cell-class zan-cell {{ isLastCell ? 'last-cell' : '' }} {{ isLink ? 'zan-cell--access' : '' }}">
 
   <view class="zan-cell__icon">
     <slot name="icon"></slot>