Allow customizing email headers

There is an existing header, X-Mailer: Atlassian-FishEye-and-Crucible, but some customers may want to add something more customizable, like a custom X-Header to the outgoing emails that can have whitelisting based on, e.g., X-Fisheye: abcdefg.

The current Fisheye and Crucible email customizations available allow customizing the email body, not the header.