Add modifiers to rewrite headers

See https://github.com/foxcpp/maddy/issues/158#issuecomment-551166366.

--
Original post:

This should not be used for relays (RFC 5321 prohibits modification of existing header fields) but is usually fine for submission servers. In case of submission, this is what people probably want when saying "replace sender" \*.
Need to think whether it may be useful for replace_rcpt too.

```
replace_sender this that {
header on # probably default
}
```

\* replacing only MAIL FROM command argument (as currently done by replace_sender) can invalidate DMARC alignment, btw.