Feature/mailchimp autosubscribe

@madflojo this isn't 100% ready yet, but I figured it would be easier to have the discussion here where the code is.  Here's what I still need to work on: 

1.  I can't get the YML file to work.  YML wants to parse the config[] sections so for some reason those aren't evaluated to string values before the parsing.  
2.  I added the attribute `subscribed_to_newsletter` but I'm not sure how to setup a migration for it.  Not sure if that's needed at all. 

I think that's it, let me know if anything else is needed.  thanks!