add support for sending to multiple recipients via sendmail

[monperrus]

if the to config contains multiple emails such as x@foo.com,y@bar.com

[Jamstah]

Does this also work for the smtp method? That field should probably work the same for both mechanisms.

This PR is missing documentation (although the doc doesn't mention sendmail at all), and an entry in the changelog.

[monperrus]

Does this also work for the smtp method?

Yes, it already works for SMTP

This PR is missing documentation and an entry in the changelog.

Done.

Thanks!

[monperrus]

Thanks for the review, applied the changes.

[monperrus]

thank you!