refactor: explicitly load cssnano on usage (webpack#472)

Author: jwalton

lib/processCss.js

@@ -13,7 +13,6 @@ var localByDefault = require("postcss-modules-local-by-default");
 var extractImports = require("postcss-modules-extract-imports");
 var modulesScope = require("postcss-modules-scope");
 var modulesValues = require("postcss-modules-values");
-var cssnano = require("cssnano");
 
 var parserPlugin = postcss.plugin("css-loader-parser", function(options) {
 	return function(css) {
@@ -141,7 +140,7 @@ module.exports = function processCss(inputSource, inputMap, options, callback) {
 	var minimize = typeof forceMinimize !== "undefined" ? !!forceMinimize : options.minimize;
 
 	var customGetLocalIdent = query.getLocalIdent || getLocalIdent;
-	
+
 	var parserOptions = {
 		root: root,
 		mode: options.mode,
@@ -179,6 +178,7 @@ module.exports = function processCss(inputSource, inputMap, options, callback) {
 	]);
 
 	if(minimize) {
+		var cssnano = require("cssnano");
 		var minimizeOptions = assign({}, query.minimize);
 		["zindex", "normalizeUrl", "discardUnused", "mergeIdents", "reduceIdents", "autoprefixer"].forEach(function(name) {
 			if(typeof minimizeOptions[name] === "undefined")