Add script that builds the demo based on `tests/expect.json`

Add a Node.js script that builds the demo based on `tests/expect.json`. To run it, use:

```bash
$ npm run build-demo
```

…or…

```bash
$ node scripts/build-demo.js
```

This is an alternate solution to #11 that I just wanted to show. Feel free to close this if the current solution is preferred.