[Doc] add Tab preview (#501)

docs/src/Preview.vue

@@ -35,6 +35,7 @@ const MAP = {
   steps: 'steps-201808092138.png',
   switch: 'switch-201808092138.png',
   'switch-cell': 'switch-cell-201808221124.png',
+  tab: 'tab-201809051457.png',
   tag: 'tag-201808092138.png',
   tabbar: 'tabbar-201808160922.png',
   toast: 'toast-201808191046.png',

example/pages/tab/index.wxml

@@ -48,7 +48,7 @@
       wx:key="index"
       title="{{ '标签' + item }}"
     >
-      <view class="content">
+      <view class="content-2">
         {{ '内容' + item }}
       </view>
     </van-tab>

example/pages/tab/index.wxss

@@ -2,3 +2,7 @@
   padding: 20px;
   background-color: #fff;
 }
+
+.content-2 {
+  padding: 20px;
+}

packages/button/index.js

@@ -1,6 +1,11 @@
 import buttonBehaviors from '../behaviors/button';
 import classnames from '../common/classnames';
 
+const booleanProp = {
+  type: Boolean,
+  observer: 'setClasses'
+};
+
 Component({
   options: {
     addGlobalClass: true
@@ -21,26 +26,11 @@ Component({
       value: 'normal',
       observer: 'setClasses'
     },
-    plain: {
-      type: Boolean,
-      observer: 'setClasses'
-    },
-    disabled: {
-      type: Boolean,
-      observer: 'setClasses'
-    },
-    loading: {
-      type: Boolean,
-      observer: 'setClasses'
-    },
-    block: {
-      type: Boolean,
-      observer: 'setClasses'
-    },
-    square: {
-      type: Boolean,
-      observer: 'setClasses'
-    }
+    plain: booleanProp,
+    block: booleanProp,
+    square: booleanProp,
+    loading: booleanProp,
+    disabled: booleanProp
   },
 
   attached() {