Add message and message_out_keys parameter, and subject_out_keys

Hi @u-ichi -san, 

I added `message`, `message_out_keys`, and `subject_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: %s
      subject_out_keys target_tag
      message %s %s\n%s
      message_out_keys target_tag,pattern,value
    </match>

`\n` is replaced with a return code in `message`. Then, Email is sent like

    From: SOURCE
    To: DEST1,DEST2,DEST3
    Subject: SUBJECT: #{target_tag}
    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.