inputs/s3.rb: Add config flag that adds S3 plugin information to events

When configured to do so, the plugin will also add the following information into the event:

    event['s3_object_key'] = @object_key
    event['s3_bucket_name'] = @bucket
    event['s3_bucket_region'] = @region_endpoint

I have many applications logging into the same bucket, which are may or may no be of the same type, and I cannot always do some filtering/add fields upstream.
By having the s3 object key, I can do filtering on the logstash reader side.

Of course, these fields can be removed after the fact by current logstash functionality.