feat: add support for multiple Reply-To addresses

[nolotz]

Hello @jhillyerd and enmime maintainers,

I've been working on a feature enhancement for the enmime library to add support for multiple reply-to addresses, as described in RFC 5322, section 3.6.2. I believe this functionality will improve the flexibility of the library and provide users with the ability to correctly handle multiple reply-to addresses as per the email specification.

Here is a brief overview of the changes I've made in this commit:

• Updated the MailBuilder struct to store multiple reply-to addresses.
• Modified the Build function to correctly write multiple reply-to addresses.
• Updated the GetReplyTo function to return the first email in the list for backward compatibility, and added a new GetReplyToAddrs function to return a slice copy of all addresses.
• Updated the existing ReplyTo function to support backward compatibility by handling single email addresses while using the new reply-to address list, and added a new ReplyToAddrs function to support setting the whole reply-to list.
• Added new tests to validate the proper handling of multiple reply-to addresses.

Thank you for your time and consideration. I'm looking forward to your feedback and the opportunity to contribute to this project.

[jhillyerd]

Thanks, good idea

[jhillyerd]

Thanks!