mailer.js: implement send_mail(), using npm module nodemailer, documentation:

 source.send_mail( configuration, [ options ] )

 Send emails using npm module nodemailer (http://www.nodemailer.com/). The source dataflow
 contains emails to send with the following attributes, for more information, and more
 options, check nodemailer's documentation:
   - from: The e-mail address of the sender. All e-mail addresses can be plain
       sender@server.com or formatted Sender Name <sender@server.com>
   - to: Comma separated list or an array of recipients e-mail addresses that will appear
       on the To: field
   - cc: Comma separated list or an array of recipients e-mail addresses that will appear
       on the Cc: field
   - bcc: Comma separated list or an array of recipients e-mail addresses that will appear
       on the Bcc: field
   - subject: The subject of the e-mail
   - text: The plaintext version of the message
   - html: The HTML version of the message

 Parameters:
   - configuration: (Pipelet) configuration options for nodemailer. Attributes are:
     - transport: (String) defaults to 'SMTP', other valid transports are:
        - 'SES' for Amazon Simple Email Service
        - 'Sendmail'
     - transport_options: (Object) as defined in nodemailer's documentation, e.g. for
         SMTP:
           - service: e.g. "Gmail"
           - auth: (Object):
             - user: user's gmail address
             - pass: user's password

   - options: (Object) optional attributes: