Added universal configuration example (#991)

* Added universal configuration example

* Make example more clear

examples/with-universal-configuration/.babelrc

@@ -0,0 +1,8 @@
+{
+  "presets": [
+    "next/babel"
+  ],
+  "plugins": [
+    ["transform-define", "./env-config.js"]
+  ]
+}

examples/with-universal-configuration/README.md

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+
+# With universal configuration
+
+## How to use
+
+Download the example [or clone the repo](https://github.com/zeit/next.js):
+
+```bash
+curl https://codeload.github.com/zeit/next.js/tar.gz/master | tar -xz --strip=2 next.js-master/examples/basic-css
+cd basic-css
+```
+
+Install it and run:
+
+```bash
+npm install
+npm run dev
+```
+
+Deploy it to the cloud with [now](https://zeit.co/now) ([download](https://zeit.co/download))
+
+```bash
+now
+```
+
+## The idea behind the example
+
+This example show how to set custom environment variables for your application based on NODE_ENV using [transform-define](https://github.com/FormidableLabs/babel-plugin-transform-define).

examples/with-universal-configuration/env-config.js

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+const prod = process.env.NODE_ENV === 'production'
+
+module.exports = {
+  'BACKEND_URL': prod ? 'https://api.example.com' : 'https://localhost:8080'
+}

examples/with-universal-configuration/package.json

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+{
+  "name": "with-env",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "next": "^2.0.0-beta"
+  },
+  "devDependencies": {
+    "babel-plugin-transform-define": "^1.2.0"
+  },
+  "author": "",
+  "license": "ISC"
+}

examples/with-universal-configuration/pages/index.js

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+/* global BACKEND_URL */
+
+export default () => (
+  <div>Loading data from { BACKEND_URL }</div>
+)