elasticsearch_http: Use source._id automatically

This is an alternative to using document_id option.  Better suited to diverse deployments when some sources provide unique id and others don't.
@zde I think I understand what you're asking, but I can't recall anyone using a field named "_id" as the document id to elasticsearch, so given I don't think this is common, I am not sure this will be a useful default. The `_` prefix is a bit weird and seems unnecessary.

If the problem is that you only specify the document id in some, but not all, events, you can use conditionals:

```
output {
  if [_id] {
    elasticsearch { 
      document_id => "%{_id}"
      ... 
    }
  } else {
    elasticsearch { ... }
  }
}