Change default value for double-opt-in to YES to match the mailchimp website default value.

I think it's important to focus on list quality, so I guess I happen to agree with this:
https://blog.mailchimp.com/double-opt-in-vs-single-opt-in-stats/

Better list quality, lower unsubscribe rate, more engagement, and…yes…probably not quite as many people. But I think it's better this way.

Also, it seems nicer to match MailChimp's default on their website. Most people don't modify the default values, and this one seems just a little dangerous for people who haven't read up on the reasoning behind it. :)