* documentation updates

* use SimpleSend() & SMTPClientConfig in email_test.go
* updated settings format in email_test_settings.json

- variadic MSG parameter for `SMTPClientConfig.SimpleSend()`

* message by pointer in `Client.Send()`

* created public `SMTPClientConfig` struct
* created `SMTPClientConfig.SimpleSend()` for easier access to SMTP logging and network timeouts

* message by pointer in `Client.Send()`

* created public `SMTPClientConfig` struct
* created `SMTPClientConfig.SimpleSend()` for easier access to SMTP logging and network timeouts