fix postcss plugin growing over rebuilds

Author: yyx990803

lib/style-rewriter.js

@@ -44,7 +44,9 @@ module.exports = function (id, css, scoped) {
   if (val) {
     return Promise.resolve(val)
   } else {
-    var plugins = options.postcss || []
+    var plugins = options.postcss
+      ? options.postcss.slice()
+      : []
     // scoped css rewrite
     if (scoped) {
       plugins.push(addId)