Use the postcss-webpack-loader

Use postcss-webpack-loader instead of webpack-cli to process the scss. Doing so
mostly means that we avoid the problem that webpack-dev-server fails to start
if we haven't already built the CSS. (It also simplifies package.json somewhat,
which is no bad thing)

postcss.config.js

@@ -0,0 +1,13 @@
+module.exports = {
+    plugins: [
+        require("postcss-import")(),
+        require("autoprefixer")(),
+        require("postcss-simple-vars")(),
+        require("postcss-extend")(),
+        require("postcss-nested")(),
+        require("postcss-mixins")(),
+        require("postcss-strip-inline-comments")(),
+    ],
+    "parser": "postcss-scss",
+    "local-plugins": true,
+};