message: [Doc] support hide demo nav
diff: <template>
  <div>
  <van-nav-bar
  - v-show="title"
  + v-show="title && showNav"
  class="van-doc-nav-bar"
  :title="title"
  left-arrow
  </template>
  <script>
  +function getQueryString(name) {
  + const arr = location.search.substr(1).split('&');
  + for (let i = 0, l = arr.length; i < l; i++) {
  + const item = arr[i].split('=');
  + if (item.shift() === name) {
  + return decodeURIComponent(item.join('='));
  + }
  + }
  + return '';
  +}
  +
  export default {
  computed: {
  title() {
  @@ -30,7 +41,13 @@ export default {
  },
  demoLink() {
  - return `https://github.com/youzan/vant/blob/dev/packages/${this.$route.meta.path}/demo/index.vue`;
  + return `https://github.com/youzan/vant/blob/dev/packages/${
  + this.$route.meta.path
  + }/demo/index.vue`;
  + },
  +
  + showNav() {
  + return getQueryString('hide_nav') !== '1';
  }
  },
  