feat: add bcc and cc specifications

[qroques]

Hi @rpkamp , I recently created my own specification as I had to assert Bcc recipients. This is why I submit this pull request.

I didn't find any contribution guide so let me know if I should adjust it.

[rpkamp]

You can't check for BCC as BCC is not a header, it is sent out as separate emails. If it were in the header it woudn't be a Blind Carbon Copy :slightly_smiling_face:

So 1 recipient + 2 BCC = 3 emails sent, one to each