Add extension option, swap out console.log for debug

I've add an option to specify the locale file extension and made it `.json` by default, e.g.

```javascript
i18n.configure({
  locales: ['en', 'de'],
  extension: '.js' // keep old extension
});
```

I've also swapped out all the calls to `console.log` to the standard `debug` module, so instead of specifying `debug: true` to view the debug output you:

```
~/app $ export DEBUG=i18n; node app.js
```