Rake task to export a publication's html attachments
Rake task to export a publication's html attachments

For use with the MHRA alert importer in specialist publisher. Can
probably be used for other things too.

Example usage:

```
$ bundle exec rake export:html_attachments[safety-information-from-manufacturers-field-safety-notices] > ~/safety-information-from-manufacturers-field-safety-notices.json
```

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall