Add Autoprefixer

Apprently it got the axe when moving to Gulp. Oops.

postcss.config.js

@@ -1,9 +1,11 @@
 const postcssPresetEnv = require('postcss-preset-env');
+const autoprefixer = require('autoprefixer');
 const cssnano = require('cssnano');
 
 const config = () => ({
     plugins: [
         postcssPresetEnv(),
+        autoprefixer(),
         cssnano()
     ]
 });