Add custom headers to emails

[hughbris]

It would be handy for message filtering purposes (without needing a magic subject line string) to support adding custom email headers in messages sent, e.g.

X-Webform-origin: /contact

Ideally this will be supported in form process.email YAML too.

There may be use cases apart from client message filtering.

SwiftMailer supports this, it seems.

[rhukster]

I think we would need a PR to add this kind of advanced functionality. But sounds useful.

[hughbris]

ref ee0feae

[rhukster]

?