[bugfix] Button: should be unclickavble when loading (#779)

packages/vant-css/src/button.css

@@ -27,10 +27,14 @@
     transform: translate(-50%, -50%); 
   }
 
-  &:not([disabled]):active::before {
+  &:active::before {
     opacity: .3;
   }
 
+  &--unclickable:before {
+    display: none;
+  }
+
   &--default {
     color: $button-default-color;
     background-color: $button-default-background-color;