Retain quotes on relative URLs in CSS

cssnano's normalizeUrl removes quotes from relative URLs, underminining
the fix for #11 that was made in 8aab4d7.

There's no configuration option that I can see on normalizeUrl to
disable the removal of quotes, so this commit disables normalizeUrl
entirely.

See gh-19

Author: wilkinsona

postcss.config.js

@@ -4,7 +4,9 @@ module.exports = {
   plugins: {
     "postcss-import": {},
     cssnano: {
-      preset: "default",
+      preset: ["default", {
+        normalizeUrl: false,
+      }]
     },
     "autoprefixer": {}
   },