Only send to Opt-in

[Casper-O]

In some countries you need explicit consent for sending e-mail marketing, including abandoned cart reminders.
It should also be possible in the e-mail to opt-out of these campaigns without

• Option to only send to customers that opt-in for newsletter
• Option in e-mail to opt-out for newsletter with a single click

[Casper-O]

Option to only send to customers that opt-in for newsletter:
PR #11

Now i don't know if you want this on your repo, since my editor have been running some PSR2 on the files i changed