Allow custom transformations to be defined for bulk imports.

This allows for the customization of bulk imports. This was driven out of need to include the parent in the index payload, though could extend to any bulk api options.

```ruby
transform = lambda do |article|
  {index: {_id: article.id, _parent: article.author_id, data: article.__elasticsearch__.as_indexed_json}}
end

Article.import transform: transform
```

This functionality doesn't extend to the rake import task. Will address in a separate PR.