Added config param of use of first match tag

# Added new config_param.

**An proposal. :)**

Add config_param of **use_of_first_match_tag_regexp** . 
I hope use of first match tag to ```${tag}```.

**How to use**

- fluentd.conf

  ```
  <source>
    type forward
    port 24224
  </source>
  
  <match hoge_game.*.api>
    type copy
  
    <store>
      type rewrite_tag_filter
      use_of_first_match_tag_regexp \w+\.(\w+)\.
      rewriterule1 type ^(\w+)$ api.${tag}.$1
    </store>
  
  </match>
  
  <match api.production.*>
    type file
    path /var/log/fluentd/warrior.log
  </match>
  
  <match api.development.*>
    type file
    path /var/log/fluentd/warrior-dev.log
  </match>
  ```

- fluent-cat command

  ```
  > echo '{"user_id": "1000", "name": "Keiji Matsuzaki", "type": "warrior"}' | bundle ex fluent-cat hoge_game.production.api
  > echo '{"user_id": "1000", "name": "Keiji Matsuzaki", "type": "warrior"}' | bundle ex fluent-cat hoge_game.development.api
  ```

**results**

- warrior.log (production)

  ```
  2013-10-09T18:11:26+09:00       api.production.warrior  {"user_id":"1000","name":"Richard Costner","type":"warrior"}
  ```

- warrior-dev.log

  ```
  2013-10-09T18:11:36+09:00       api.development.warrior {"user_id":"1000","name":"Mason Smith","type":"warrior"}
  ```