Added option to add the tag and timestamp to record

I've expanded the buffered version to optionally add the tag and timestamp, which are not being sent to kafka. Additionally, the update allows users to format the timestamp being added. Usage is mostly similar, just add the following conf settings to include it in the record: tag_key, time_key and time_format.

Example conf:
```
<match *.**>
  type             kafka_buffered
  brokers          <broker1_host>:<broker1_ip>,<broker2_host>:<broker2_ip>,..
  default_topic    <output topic>
  flush_interval   <flush interval (sec) :default => 60>
  buffer_type      (file|memory)
  output_data_type (json|ltsv|attr:<record name>)
  tag_key tag
  time_key timestamp
  time_format %Y-%m-%d %H:%M:%S
</match>
```