Improve CSS minification

Use CleanCSS instead of cssmin. We need to wrap CleanCSS in function to mimic cssmin output so it works with express-minify

lib/minify.js

@@ -1,8 +1,12 @@
 const minify      = require('express-minify');
 const minifyHTML  = require('express-minify-html');
+const CleanCSS    = require('clean-css');
+const cleanCSS    = new CleanCSS();
 
 module.exports = [
-  minify(),
+  minify({
+    cssmin: source => cleanCSS.minify(source).styles
+  }),
   minifyHTML({
     htmlMinifier: {
       removeComments:            true,

package.json

@@ -5,6 +5,7 @@
   "main": "index.js",
   "dependencies": {
     "ascii-chart": "^1.3.1",
+    "clean-css": "^3.4.19",
     "compression": "^1.6.2",
     "express": "^4.14.0",
     "express-minify": "^0.2.0",