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.

u-ichi / fluent-plugin-mail

Add message and message_out_keys parameter, and subject_out_keys #1