Add message and message_out_keys parameter

Hi @u-ichi -san, 

I added `message` and `message_out_keys` parameter for your plugin. These parameters enable users to specify  email message format with %s. 

The configuration becomes as belows (I also described in README):

    <match **>
      type mail
      host SMTPSERVER
      port 25
      from SOURCE
      to DEST1,DEST2,DEST3
      subject SUBJECT
      message %s %s\\n%s
      message_out_keys target_tag,pattern,value
    </match>

`\\n` is replaced with a return code. Then, Email is sent like

    Date: #{Date}
    From: SOURCE
    To: DEST1,DEST2,DEST3
    Subject: SUBJECT
    Mime-Version: 1.0
    Content-Type: text/plain; charset=utf-8

    #{target_tag} #{pattern}
    #{value}

Please notice that `message` parameter is not required, but either `message` or `out_keys` is required, now. 

Please let me know if you find something which I should fix.