Add optional nodeMailerOptions argument to sendgrid.smtp

PR for Issue #74 - lets you pass nodemailer additional options for the outbound email, such as the Message-Id.  

See https://github.com/andris9/Nodemailer#e-mail-message-fields for options.

@scottmotte, I'm not sure how to write mocks against the SMTP stuff, so sorry, there's not a test included in this PR.