Add autoprefixing to CSS pipeline (#867)

Add [autoprefixer](https://github.com/postcss/autoprefixer) as a final build step in the CSS pipeline.

lib/server/utils.js

@@ -6,6 +6,8 @@
  */
 
 const cssnano = require('cssnano');
+const autoprefixer = require('autoprefixer');
+const postcss = require('postcss');
 const path = require('path');
 const escapeStringRegexp = require('escape-string-regexp');
 
@@ -54,9 +56,18 @@ function minifyCss(cssContent) {
     .then(result => result.css);
 }
 
+function autoPrefixCss(cssContent) {
+  return postcss([autoprefixer])
+    .process(cssContent, {
+      from: undefined,
+    })
+    .then(result => result.css);
+}
+
 module.exports = {
   getSubDir,
   getLanguage,
   isSeparateCss,
   minifyCss,
+  autoPrefixCss,
 };