[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/index.vue

@@ -0,0 +1,46 @@
+<template>
+  <div :class="['van-tabbar', 'van-hairline--top-bottom', { 'van-tabbar--fixed': fixed }]">
+    <slot></slot>
+  </div>
+</template>
+
+<script>
+export default {
+  name: 'van-tabbar',
+
+  data() {
+    return {
+      items: []
+    };
+  },
+
+  props: {
+    value: Number,
+    fixed: {
+      type: Boolean,
+      default: true
+    }
+  },
+
+  watch: {
+    items() {
+      this.setActiveItem();
+    },
+    value() {
+      this.setActiveItem();
+    }
+  },
+
+  methods: {
+    setActiveItem() {
+      this.items.forEach((item, index) => {
+        item.active = index === this.value;
+      });
+    },
+    onChange(active) {
+      this.$emit('input', active);
+      this.$emit('change', active);
+    }
+  }
+};
+</script>