message: [Improvement] Area: add item-height & visible-item-count props
diff: @@ -44,6 +44,8 @@ Set `columns-num` with 2, you'll have a 2 level picker.
  | title | Toolbar title | `String` | `''` | - |
  | 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` | - |
  +| visible-item-count | Count of visible columns | `Number` | `5` | - |
  ### Event
  