Add postpone command

This command uses the Postpone folder from the account config to save
messages to. Messages are saved as though they were sent so have a valid
'to' recipient address and should be able to be read back in for later
editing.

commands/msg/unsubscribe.go

@@ -90,6 +90,7 @@ func unsubscribeMailto(aerc *widgets.Aerc, u *url.URL) error {
 	}
 	composer, err := widgets.NewComposer(
 		aerc,
+		acct,
 		aerc.Config(),
 		acct.AccountConfig(),
 		acct.Worker(),