Exposing a hook to other extensions

What are your thoughts on providing a post-sync hook? I'd like to rig up [Middleman CloudFront](https://github.com/andrusha/middleman-cloudfront) to automatically invalidate whatever has been updated, a la:

```ruby
after_s3_sync do |destination_paths|
  invalidate destination_paths[:updated]
end
```

Is this a good way to go?
That said.. we only use ```@app``` in one place. It should be ok to inline getting the instance with the run_hook call.