Add JS wrapper to CSS to support Serverless target

Resolves #281

src/css/index.js

@@ -0,0 +1,10 @@
+// To support serverless targets (which don't work if you try to read in things
+// like CSS files at run time) this file is replaced in production builds with
+// a function that returns compiled CSS (embedded as a string in the function).
+import fs from 'fs'
+import path from 'path'
+
+const pathToCss = path.join(__dirname, '/index.css')
+const css = fs.readFileSync(pathToCss, 'utf8')
+
+export default () => css