add NavBar component

packages/vant-css/src/index.css

@@ -15,6 +15,8 @@
 @import './card.css';
 @import './icon.css';
 @import './loading.css';
+@import './nav-bar.css';
+@import './notice-bar.css';
 @import './popup.css';
 @import './search.css';
 @import './panel.css';
@@ -48,5 +50,4 @@
 /* business components */
 @import './coupon-list.css';
 @import './goods-action.css';
-@import './notice-bar.css';
 @import './submit-bar.css';

packages/vant-css/src/nav-bar.css

@@ -0,0 +1,60 @@
+@import './common/var.css';
+
+.van-nav-bar {
+  height: 46px;
+  position: relative;
+  user-select: none;
+  text-align: center;
+  line-height: 46px;
+  background-color: #fff;
+
+  .van-icon {
+    color: $blue;
+  }
+
+  &__arrow {
+    transform: rotate(180deg);
+
+    + .van-nav-bar__text {
+      margin-left: -20px;
+      padding-left: 25px;
+    }
+  }
+
+  &--fixed {
+    top: 0;
+    left: 0;
+    width: 100%;
+    position: fixed;
+  }
+
+  &__title {
+    font-size: 16px;
+  }
+
+  &__left,
+  &__right {
+    bottom: 0;
+    font-size: 14px;
+    position: absolute;
+  }
+
+  &__left {
+    left: 15px;
+  }
+
+  &__right {
+    right: 15px;
+  }
+
+  &__text {
+    color: $blue;
+    margin: 0 -15px;
+    padding: 0 15px;
+    display: inline-block;
+
+    &:active {
+      background-color: $active-color;
+    }
+  }
+}