Allow flag in .ini/settings for using debug mailer

[digitalresistor]

I prefer to explicitly config.include() my app's dependencies rather than relying on the .ini file to correctly include them.

It would be neat if there was a flag I could set in my settings that would cause pyramid_mailer to use pyramid_mailer.debug instead even when config.include('pyramid_mailer') is used. For right now I just spin up a fake SMTP server using Python and capture messages sent that way, but a debug mailer that sends output to stdout/logging would make debugging simpler.