[new feature] add Tabbar component (#204)

* [Document] add english document of Checkbox

* [Document] add english document of Field

* [Document] add english document of NumberKeyboard

* [bugfix] NumberKeyboard should not dispaly title when title is empty

* [Document] add english document of PasswordInput

* [Document] add english document of Radio

* [document] add english document of Switch

* [bugfix] remove redundent styles in english document

* [Document] fix details

* fix Switch test cases

* [bugfix] Swipe shouid reinitialize when item changes

* [new feature] ImagePreview reconstruct

* [new feature] add Tabbar component

test/unit/components/tabbar.vue

@@ -0,0 +1,40 @@
+<template>
+  <van-tabbar v-model="active" @change="onChange">
+    <van-tabbar-item icon="shop">
+      <span>自定义</span>
+      <img slot="icon" :src="active === 0 ? icon.active : icon.normal" />
+    </van-tabbar-item>
+    <van-tabbar-item icon="chat">标签</van-tabbar-item>
+    <van-tabbar-item icon="chat">标签</van-tabbar-item>
+    <van-tabbar-item icon="records">标签</van-tabbar-item>
+  </van-tabbar>
+</template>
+
+<script>
+import Tabbar from 'packages/tabbar';
+import TabbarItem from 'packages/tabbar-item';
+
+export default {
+  components: {
+    [Tabbar.name]: Tabbar,
+    [TabbarItem.name]: TabbarItem
+  },
+
+  data() {
+    return {
+      active: 0,
+      changeRecord: 0,
+      icon: {
+        normal: 'https://img.yzcdn.cn/public_files/2017/10/13/c547715be149dd3faa817e4a948b40c4.png',
+        active: 'https://img.yzcdn.cn/public_files/2017/10/13/793c77793db8641c4c325b7f25bf130d.png'
+      }
+    };
+  },
+
+  methods: {
+    onChange(val) {
+      this.changeRecord = val;
+    }
+  }
+};
+</script>