[improvement] Button: add round prop (#1885)

packages/button/index.vue

@@ -9,6 +9,7 @@
       {
         block,
         plain,
+        round,
         square,
         loading,
         disabled,
@@ -35,6 +36,7 @@ export default create({
     text: String,
     block: Boolean,
     plain: Boolean,
+    round: Boolean,
     square: Boolean,
     loading: Boolean,
     disabled: Boolean,