Add option to pass in Alerter config

In many cases the response messages may need to use custom markup, rather than the default template from the `blocks-alerter` module. Since the that module allows the template to be overridden, and there are other options that we may want to pass to it, we should replace the `prependMessagesTo` option with an option to pass an alerter config object to `blocks-subscribe-email` like this;

```
var mySubscribe = new SubscribeEmail({
  element: '#subscribe-form',
  service: 'universe',
  key: config.key,
  alerter: {
    prependTo: '#header',
    template: customTemplate
  }
});
```

I would move the variable declaration out of the condition, to avoid confusion.
```javascript
var messageHolder;
if (options.alerter) messageHolder = new Alerter(options.alerter);
```