Enhancement: add mailerBuilder.withTransportModeLoggingOnly() as mailer API entry point

[bbottema]

Without this, a user would be forced to start with a dummy SMTP server.

[bbottema]

7.8.0 released, now SMTP server config can be omitted in case of withTransportModeLoggingOnly:

Mailer mailer = MailerBuilder
    .withTransportModeLoggingOnly()
    .(...)
    .buildMailer();