Allow user save the docinfo information from an elasticsearch document into an event.
- Turn on the feature by setting `docinfo` option to true.
- You can select where the docinfo will be saved with the
  `docinfo_target` option and by default we use the `@metadata` field.
- If the original document contains the targetted field we will try to
  merge if the target is a hash, if it is not a hash we will raise an
  exception.

* Original code from PR #730 (author Thomas De Smedt)