[new feature] Picker: add loading prop (#619)

docs/markdown/en-US/area.md

@@ -45,6 +45,7 @@ Set `columns-num` with 2, you'll have a 2 level picker.
 | area-list | an object contains these properties: `province_list`, `city_list` and `county_list`  | `Object` | - | - |
 | columns-num | level of picker | `String`,`Number` | 3 | - |
 | item-height | Option height | `Number` | `44` | - |
+| loading | Whether to show loading prompt | `Boolean` | `false` | - |
 | visible-item-count | Count of visible columns | `Number` | `5` | - |
 
 ### Event

docs/markdown/en-US/picker.md

@@ -114,6 +114,13 @@ export default {
 };
 ```
 
+#### Loading
+When Picker columns data is acquired asynchronously, use `loading` prop to show loading prompt
+
+```html
+<van-picker :columns="columns" loading />
+```
+
 ### API
 
 | Attribute | Description | Type | Default | Accepted Values |
@@ -121,6 +128,7 @@ export default {
 | columns | Columns data | `Array` | `[]` | - |
 | show-toolbar | Whether to show toolbar | `Boolean` | `false` | - |
 | title | Toolbar title | `String` | `''` | - |
+| loading | Whether to show loading prompt | `Boolean` | `false` | - |
 | confirm-button-text | Text of confirm button | `String` | `Confirm` | - |
 | cancel-button-text | Text of cancel button | `String` | `Cancel` | - |
 | item-height | Option height | `Number` | `44` | - |