Support split of tag

**Support split of tag.**

An proposal.
This difference is split tag, use of period.

```ruby
${tag} #== hoge.fuga.moge

${tags[0]} #== hoge
${tags[1]} #== fuga
${tags[2]} #== moge
```

**fluentd.conf(sample)**

```
<source>
  type forward
  port 24244
</source>

<match app.game.*>
  type rewrite_tag_filter
  rewriterule1 server_type ^([a-z]+)$ ${tag[1]}.$1.${tag[2]}
</match>

<match game.production.api>
  type copy
  <store>
    type tdlog
    api_key hogehogehogehogehogehoge
  </store>

  <store>
    type file
    path /var/log/fluentd/production_api.log
  </store>
</match>

<match game.staging.api>
  type copy
  <store>
    type mongo
  </store>
  <store>
    type file
    path /var/log/fluentd/staging_api.log
  </store>
</match>

<match game.development.api>
  type file
  path /var/log/fluentd/development_api.log
</match>
```

**command**

```
> echo '{"user_id": "10000", "name": "hogehoge", "server_type": "development", "description": "super user"}' | bundle ex fluent-cat app.game.peach
> echo '{"user_id": "10000", "name": "hogehoge", "server_type": "production", "description": "super user"}' | bundle ex fluent-cat app.game.mario
> echo '{"user_id": "10000", "name": "hogehoge", "server_type": "staging", "description": "super user"}' | bundle ex fluent-cat app.game.luigi
```

**result**

- production_api.log

```
2013-10-11T15:25:28+09:00       game.production.mario   {"user_id":"10000","name":"hogehoge","server_type":"production","description":"super user"}
```

- staging_api.log

```
2013-10-11T15:25:48+09:00       game.staging.luigi      {"user_id":"10000","name":"hogehoge","server_type":"staging","description":"super user"}
```

- development_api.log

```
2013-10-11T15:25:08+09:00       game.development.peach  {"user_id":"10000","name":"hogehoge","server_type":"development","description":"super user"}
```