fix postcss plugin list growing over rebuilds

Author: yyx990803

lib/style-rewriter.js

@@ -36,7 +36,9 @@ module.exports = function (css, map) {
   var query = loaderUtils.parseQuery(this.query)
   var options = this.options.vue || {}
   var autoprefixOptions = options.autoprefixer
-  var plugins = options.postcss || []
+  var plugins = options.postcss
+    ? options.postcss.slice() // make sure to copy it
+    : []
 
   // scoped css
   if (query.scoped) {