Add support for custom ID to be used by Java Mail on MimeMessage

[bbottema]

See #77 for requirement details.

Need to solve this properly though. Email is the Simple Java Mail abstraction to make the underlying API simple. Exposing MimeMessage in Email would be doing the opposite.

Instead Email will have an ID field, which will be used if provided.

Note though that the ID needs to be conform https://tools.ietf.org/html/rfc5322#section-3.6.4

[bbottema]

Released in 4.2.3

Email email = new Email();
email.setId("<123@456>");

Or

new EmailBuilder()
   .id("<123@456>")
   .build();