Add support in Address structure for BCC and CC

[ewandennis]

@Sam23D Did you have an idea of how this would work?

It's an interesting one since the SparkPost API itself doesn't have separate support for CC andBCC`. It's really just a convention where:

• we add all the CC and BCC recipients to the recipients list
• we use a single header_to for all recipients, listing the To recipients
• we set a custom CC header to name all the CC recipients

If we added CC and BCC fields to Address, we'd presumably then use those steps to transform the request which before sending it on the wire?