[Improvement] Button: add plain prop (#1444)

packages/button/en-US.md

@@ -17,6 +17,14 @@ Vue.use(Button);
 <van-button type="danger">Danger</van-button>
 ```
 
+#### Plain
+
+```html
+<van-button type="default" plain>Default</van-button>
+<van-button type="primary" plain>Primary</van-button>
+<van-button type="danger" plain>Danger</van-button>
+```
+
 #### Size
 
 ```html 
@@ -48,21 +56,6 @@ Use `tag` prop to custom button tag
 </van-button>
 ```
 
-#### Action Button
-
-```html 
-<van-button type="primary" bottom-action>Button</van-button>
-
-<van-row>
-  <van-col span="12">
-    <van-button bottom-action>Button</van-button>
-  </van-col>
-  <van-col span="12">
-    <van-button type="primary" bottom-action>Button</van-button>
-  </van-col>
-</van-row>
-```
-
 ### API
 
 | Attribute | Description | Type | Default |
@@ -72,10 +65,10 @@ Use `tag` prop to custom button tag
 | text | Text | `String` | - |
 | tag | Tag | `String` | `button` |
 | native-type | Native Type Attribute | `String` | `''` |
-| disabled | Whether disable button | `Boolean` | `false` |
+| plain | Whether to be plain button | `Boolean` | `false` |
+| disabled | Whether to disable button | `Boolean` | `false` |
 | loading | Whether show loading status | `Boolean` | `false` |
 | block | Whether to set display block | `Boolean` | `false` |
-| bottom-action | Whether to be action button | `Boolean` | `false` |
 
 ### Event