Add a 'KeepRejects' option that disables auto-cleanup (#49)

Add a 'KeepRejects' option that disables auto cleanup of the rejects mailbox.

config/config.go

@@ -300,6 +300,7 @@ type Account struct {
 		Period time.Duration `sconf-doc:"How long unique values are accepted after generating, e.g. 12h."` // todo: have a reasonable default for this?
 	} `sconf:"optional" sconf-doc:"If configured, messages classified as weakly spam are rejected with instructions to retry delivery, but this time with a signed token added to the subject. During the next delivery attempt, the signed token will bypass the spam filter. Messages with a clear spam signal, such as a known bad reputation, are rejected/delayed without a signed token."`
 	RejectsMailbox     string `sconf:"optional" sconf-doc:"Mail that looks like spam will be rejected, but a copy can be stored temporarily in a mailbox, e.g. Rejects. If mail isn't coming in when you expect, you can look there. The mail still isn't accepted, so the remote mail server may retry (hopefully, if legitimate), or give up (hopefully, if indeed a spammer). Messages are automatically removed from this mailbox, so do not set it to a mailbox that has messages you want to keep."`
+	KeepRejects	bool `sconf:"optional" sconf-doc:"Don't automatically delete mail in the RejectsMailbox listed above. This can be useful, e.g. for future spam training."`
 	AutomaticJunkFlags struct {
 		Enabled              bool   `sconf-doc:"If enabled, flags will be set automatically if they match a regular expression below. When two of the three mailbox regular expressions are set, the remaining one will match all unmatched messages. Messages are matched in the order specified and the search stops on the first match. Mailboxes are lowercased before matching."`
 		JunkMailboxRegexp    string `sconf:"optional" sconf-doc:"Example: ^(junk|spam)."`