Allow multiple To addresses in SimpleSendableMail
Allow multiple To addresses in SimpleSendableMail

Because the struct internally stores a `Vec<String>` it would be nice to be able to construct messages with multiple To addresses.
This is in its current form a breaking API change, so feel free to change the way it's implemented.
lettre/lettre,amousset/rust-smtp,amousset/lettre,amousset/lettre