Add support for cc/bcc

[zaporylie]

It took me a couple hours to understand how Cc and Bcc works with Transmissions. Would be nice to simplify it as you did for python library.

Proposal:

• add cc and bcc fields to transmission json.
• add cc and bcc automatically to recipient list on sdk level
• ensure customHeaders['Cc'] contains all Cc recipients

ref. https://support.sparkpost.com/customer/portal/articles/1948014-how-to-add-cc-and-bcc-to-emails

[richleland]

We're going to tackle this as a part of the 2.x refactor.

[richleland]

Will be resolved in 2.x with #122.

[richleland]

2.0 has been released: https://packagist.org/packages/sparkpost/sparkpost