sup-tweak-labels: sync back if specified

RE: #295

sup-tweak will only sync back if

1 - :sync_back_to_maildir is set to true (in `~/.sup/config.yml`)
2 - sup-tweak was ran with the option `--sync-back`
3 - The modified email comes from a source with sync_back enabled (in `~/.sup/sources.yml`)