support out stream for css extraction

Author: yyx990803

README.md

@@ -287,11 +287,13 @@ Via API:
 browserify('./main.js')
   .transform('vueify')
   .plugin('vueify/plugins/extract-css', {
-    out: 'dist/bundle.css'
+    out: 'dist/bundle.css' // can also be a WritableStream
   })
   .bundle()
 ```
 
+This only works for vueify 9+. For Vue 1.x / vueify 8.x you can use [vueify-extract-css](https://github.com/rawcreative/vueify-extract-css).
+
 ## Compiler API
 
 The compiler API (originally `vue-component-compiler`) is also exposed:

plugins/extract-css.js

@@ -8,15 +8,18 @@ module.exports = function (b, opts) {
 
   var styles = Object.create(null)
   var outPath = opts.out || opts.o || 'bundle.css'
-  if (typeof outPath === 'function') {
-    outPath = outPath()
-  }
 
   b.on('bundle', function (bs) {
     bs.on('end', function () {
-      fs.writeFile(outPath, Object.keys(styles)
+      var css = Object.keys(styles)
         .map(function (file) { return styles[file] })
-        .join('\n'))
+        .join('\n')
+      if (typeof outPath === 'object' && outPath.write) {
+        outPath.write(css)
+        outPath.end()
+      } else if (typeof outPath === 'string') {
+        fs.writeFile(outPath, css)
+      }
     })
   })