[MlMt] Exclude from Backup / Time Machine

[Randy Bush]

> Everything specific to MM is in ~/Library/Application Support/MailMate
> except for a .plist file in ~/Library/Preferences/

that is what i thought until i found

```
ryuu.rg.net:/Users/randy> ls -l ~/com.freron.MailMate/Messages.noindex/
total 0
drwx------@ 6 randy  staff  192 Sep  5 09:20 IMAP/
```

randy