add logstash_prefix option

Hi.
[Kibana3](http://three.kibana.org/) can use specified logstash indexes (ex. mylogstash-YYMMDD).

So I want to specify logstash index prefix like below:
```
<match my.logs>
  type elasticsearch
  include_tag_key true
  tag_key _key
  logstash_fomat true
  logstash_prefix mylogs
</match>
```

This may be the same issue as [#10](https://github.com/uken/fluent-plugin-elasticsearch/pull/10).