add mail-header config and add mode to send all log message

[yamamo-i]

I added mail_header config and new message sending mode for fluent-plugin-mail.

• mail_header config * reply_to * return_path \ auth_type
• message sending mode is all log message sending.

p.s. I'm no good at English. so. My English is wrong...maybe.

[sonots]

• reply_to
• return_path
• auth_type

looks okay if there are tests, but what is message_out_record? Can you write the use-case of it in details?

I recommend you to split this pull request to two.

[sonots]

I will close this because of no response. Please send another pull request if you want this feature again.