Replace Subject Modifier

[twobiers]

Use case

I would like to be able to replace subjects based on regexp.
I'm in a multi-environment setup, and like to prefix E-Mails for non-production E-Mails with the environment.
So for instance, the subject of an E-Mail from a Test environment will get prefixed with [TEST]

Your idea for a solution

The implementation can be similar to the replace_rcpt / replace_sender modifier.

I'm willing to provide a PR for that

• I'm willing to help with the implementation