[new feature] NoticeBar: support use inner icon (#2305)

packages/notice-bar/en-US.md

@@ -14,7 +14,7 @@ Vue.use(NoticeBar);
 ```html
 <van-notice-bar
   text="Only those who have the patience to do simple things perfectly ever acquire the skill to do difficult things easily."
-  left-icon="//img.yzcdn.cn/vant/volume.png"
+  left-icon="volume-o"
 />
 ```
 
@@ -46,11 +46,10 @@ Vue.use(NoticeBar);
 | delay | Animation delay (s) | `Number` | `1` |
 | speed | Scroll speed (px/s) | `Number` | `50` |
 | scrollable | Whether to scroll content | `Boolean` | `true` |
-| left-icon | Image url of left icon | `String` | - |
+| left-icon | Left Icon | `String` | - |
 | color | Text color | `String` | `#f60` |
 | background | Background color | `String` | `#fff7cc` |
 
-
 ### Event
 
 | Event | Description | Arguments |