Regression bug: Email parameter missing in CustomMailer interface

While implementing #383, an unnecessary [change](https://github.com/bbottema/simple-java-mail/blob/46f5528d850e53c6b3d263a62b0d08cc9a8d280a/modules/core-module/src/main/java/org/simplejavamail/api/mailer/CustomMailer.java#L21) was only partially reverted. This left the CustomMailer interface with a missing Email instance parameter. This should be restored.