* [ADD] new module base_mail_bcc
* [IMP] correct typos in README, change name of configuration parameter
* [REF] cleaned up code
* [IMP] base_mail_bcc: replicate exact API of super method
* [IMP] base_mail_bcc: add test code
* [REF] base_mail_bcc: code clean-up
* [IMP] base_mail_bcc: improve test coverage by triggering BCC-append case