`cdn_invalidate` helper that accepts a list of files

Is this something you'd want? I'm using it with an [`after_s3_sync`][middleman-s3_sync] hook in my `config.rb`:

```ruby
after_s3_sync do |files_by_status|
  cdn_invalidate files_by_status[:updated]
end
```

  [middleman-s3_sync]: https://github.com/fredjean/middleman-s3_sync