Added an opt-in flag to optionally include bcc headers (#15)

* New: added an opt-in flag to optionally include bcc headers

* default writeBccHeader to false

Changes the default MailYak behaviour to NOT write the BCC header to the
MIME body - this is a breaking change for some (hopefully small) subset
of users but is by far the safest "out-the-box" setting for most users.

For more information, see #14

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Cover insane edge cases