Add PostCSS to build process

I'm not sure why we need this, but the Bootstrap build uses it and
I would like to follow their lead.

See: https://github.com/twbs/bootstrap/blob/v5.0.0-alpha3/package.json

build/postcss.config.js

@@ -0,0 +1,16 @@
+'use strict'
+
+module.exports = ctx => ({
+  map: ctx.file.dirname.includes('examples') ?
+    false :
+    {
+      inline: false,
+      annotation: true,
+      sourcesContent: true
+    },
+  plugins: {
+    autoprefixer: {
+      cascade: false
+    }
+  }
+})