[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

packages/tabbar-item/index.vue

@@ -0,0 +1,49 @@
+<template>
+  <div :class="['van-tabbar-item', { 'van-tabbar-item--active': active }]" @click="onClick">
+    <div :class="['van-tabbar-item__icon', { 'van-tabbar-item__icon-dot': dot }]">
+      <slot name="icon">
+        <van-icon v-if="icon" :name="icon" />
+      </slot>
+    </div>
+    <div class="van-tabbar-item__text">
+      <slot></slot>
+    </div>
+  </div>
+</template>
+
+<script>
+import Icon from '../icon';
+
+export default {
+  name: 'van-tabbar-item',
+
+  components: {
+    [Icon.name]: Icon
+  },
+
+  props: {
+    icon: String,
+    dot: Boolean
+  },
+
+  data() {
+    return {
+      active: false
+    };
+  },
+
+  beforeCreate() {
+    this.$parent.items.push(this);
+  },
+
+  destroyed() {
+    this.$parent.items.splice(this.$parent.items.indexOf(this), 1);
+  },
+
+  methods: {
+    onClick() {
+      this.$parent.onChange(this.$parent.items.indexOf(this));
+    }
+  }
+};
+</script>