hostnameの値を取得するkeyに"hostname"という単語を利用できるようにする


`fluent-mixin-config-placeholders`を利用した`fluent-plugin-record-modifier`で  

```
hostname ${hostname}
```

を利用しようとしたところ、ホスト名を取得するkeyに`hostname`という単語を使用すると
意図した結果にならなかったので、このpull-reqを送ります。
このpull-reqを反映すると`hostname`

```
hostname ${hostname}
```
は利用できるようになりますが、筋が悪いかもしれません。
ご確認いただけますでしょうか。

## fluentd.conf

```
<source>
  type forward
  port 24224
</source>

<match modifiered.**>
  type copy
  <store>
    type stdout
  </store>
</match>

<match **>
  type copy
  <store>
    type stdout
  </store>
  <store>
    type record_modifier
    hostname ${hostname}
    tag modifiered
  </store>
</match>
```

## 変更前

```
$ echo '{"message" : "hoge"}' | fluent-cat tagname
$ echo '{"message" : "hoge"}' | fluent-cat tagname.hoge
```

```
$ fluentd -c fluentd.conf2014-01-16 23:09:49 +0900 [info]: starting fluentd-0.10.40
2014-01-16 23:09:49 +0900 [info]: reading config file path="fluentd.conf"
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-mixin-config-placeholders' version '0.2.3'
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-plugin-forest' version '0.2.2'
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-plugin-mongo' version '0.7.1'
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-plugin-record-modifier' version '0.1.2'
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-plugin-record-reformer' version '0.2.1'
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-plugin-redshift' version '0.0.2'
2014-01-16 23:09:50 +0900 [info]: gem 'fluent-plugin-s3' version '0.3.4'
2014-01-16 23:09:50 +0900 [info]: gem 'fluentd' version '0.10.40'
2014-01-16 23:09:50 +0900 [info]: using configuration file: <ROOT>
  <source>
    type forward
    port 24224
  </source>
  <match modifiered.**>
    type copy
    <store>
      type stdout
    </store>
  </match>
  <match **>
    type copy
    <store>
      type stdout
    </store>
    <store>
      type record_modifier
      hostname ${hostname}
      tag modifiered
    </store>
  </match>
</ROOT>
2014-01-16 23:09:50 +0900 [info]: adding source type="forward"
2014-01-16 23:09:50 +0900 [info]: adding match pattern="modifiered.**" type="copy"
2014-01-16 23:09:50 +0900 [info]: adding match pattern="**" type="copy"
2014-01-16 23:09:50 +0900 [info]: listening fluent socket on 0.0.0.0:24224
2014-01-16 23:09:54 +0900 tagname: {"message":"hoge"}
2014-01-16 23:09:54 +0900 modifiered: {"message":"hoge","hostname":"${hostname}"}
2014-01-16 23:09:57 +0900 tagname.hoge: {"message":"hoge"}
2014-01-16 23:09:57 +0900 modifiered: {"message":"hoge","hostname":"${hostname}"}
```

## 変更後

```
$ fluentd -c fluentd.conf
2014-01-16 23:06:17 +0900 [info]: starting fluentd-0.10.40
2014-01-16 23:06:17 +0900 [info]: reading config file path="fluentd.conf"
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-mixin-config-placeholders' version '0.2.3'
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-plugin-forest' version '0.2.2'
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-plugin-mongo' version '0.7.1'
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-plugin-record-modifier' version '0.1.2'
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-plugin-record-reformer' version '0.2.1'
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-plugin-redshift' version '0.0.2'
2014-01-16 23:06:17 +0900 [info]: gem 'fluent-plugin-s3' version '0.3.4'
2014-01-16 23:06:17 +0900 [info]: gem 'fluentd' version '0.10.40'
2014-01-16 23:06:17 +0900 [info]: using configuration file: <ROOT>
  <source>
    type forward
    port 24224
  </source>
  <match modifiered.**>
    type copy
    <store>
      type stdout
    </store>
  </match>
  <match **>
    type copy
    <store>
      type stdout
    </store>
    <store>
      type record_modifier
      hostname ${hostname}
      tag modifiered
    </store>
  </match>
</ROOT>
2014-01-16 23:06:17 +0900 [info]: adding source type="forward"
2014-01-16 23:06:17 +0900 [info]: adding match pattern="modifiered.**" type="copy"
2014-01-16 23:06:17 +0900 [info]: adding match pattern="**" type="copy"
2014-01-16 23:06:17 +0900 [info]: listening fluent socket on 0.0.0.0:24224
2014-01-16 23:06:21 +0900 tagname: {"message":"hoge"}
2014-01-16 23:06:21 +0900 modifiered: {"message":"hoge","hostname":"testing.local"}
2014-01-16 23:06:54 +0900 tagname.hoge: {"message":"hoge"}
2014-01-16 23:06:54 +0900 modifiered: {"message":"hoge","hostname":"testing.local"}
```