Toggle subscription for new posts.  

Addresses #757 

This should be all we need to do on the JP side.  Since disrupting Sync is not an option (<a href="https://github.com/Automattic/jetpack/pull/1267">#</a>), what the plan is here is to send over a boolean to .com as [module_custom_data] and check there before sending out the subscription email.  

I've been having trouble testing changes when using my sandbox as Jetpack's API, but I would think we need to put a stop to `send_post()` in subscription-mailer.php