add merge tags for custom subject line

[nealrs]

This package is awesome -- but adding support for merge tags in the subject line would be crazy-awesome! 🚀

[zachlatta]

Sorry about taking so long to get to this! Finally getting through some of my (unfortunately) long backlog.

I certainly agree – templating in the subject line would be great. Would you be interested in submitting a PR for this? I don't think it'd be too big of a change.