Allow multiple route callbacks

The expressjs methods `app.get`, `app.post` etc. support multiple callbacks.

I use this feature to ensure the user is authenticated on specific routes:
```
app.get('/secure-page', ensureAuthenticated, securePageCallback);
```

The `i18n.addRoute` method only accepted only **one** callback, so I improved its functionality to work with my setup. I think this could be useful to everyone.