[5.8] Support headers with notification mail message

[dwightwatson]

This extends the MailMessage used when creating basic mail notifications to support adding headers. It looks like this has been discussed before (see laravel/ideas#475 and in a couple of other places) but there doesn't appear to be a stable solution, apart from creating mailables and returning those instead.

The common use-case for custom email headers is that they can be used for tracking purposes. For example, Mailgun uses headers to help breakdown emails sent for analytics purposes.

[dwightwatson]

I think a better approach here might be to fall in line with Mailable and provide callbacks that give more control over the final message. I'll open another PR that does that instead.

[Cyrille37]

Hello. Thanks again @dwightwatson for your work. With this PR it was very easy to understand and extends a Laravel 5.5 to add header feature to MailMessage. Thanks & cheers :-)