Account for before not being an array sometimes

Author: saiichihashimoto

index.js

@@ -114,7 +114,7 @@ module.exports = function (browserify, options) {
 
   var postcssBefore = options.postcssBefore || options.before || [];
   var postcssAfter = options.postcssAfter || options.after || [];
-  plugins = postcssBefore.concat(plugins).concat(postcssAfter);
+  plugins = (Array.isArray(postcssBefore) ? postcssBefore : [postcssBefore]).concat(plugins).concat(postcssAfter);
 
   // load plugins by name (if a string is used)
   plugins = plugins.map(function requirePlugin (name) {