[new feature] Button: add loading-size prop (#2854)

packages/button/en-US.md

@@ -72,6 +72,7 @@ Vue.use(Button);
 | disabled | Whether to disable button | `Boolean` | `false` |
 | loading | Whether show loading status | `Boolean` | `false` |
 | loading-text | Loading text | `String` | - |
+| loading-size | Loading icon size | `String` | `20px` |
 | url | Link URL | `String` | - |
 | to | Target route of the link, same as to of `vue-router` | `String | Object` | - |
 | replace | If true, the navigation will not leave a history record | `String` | `false` |