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

packages/button/index.vue

@@ -8,6 +8,7 @@
       size,
       {
         block,
+        plain,
         loading,
         disabled,
         unclickable: disabled || loading,
@@ -32,6 +33,7 @@ export default create({
   props: {
     text: String,
     block: Boolean,
+    plain: Boolean,
     loading: Boolean,
     disabled: Boolean,
     nativeType: String,