Add ability to export from gmvault's internal storage to standard formats

Fixes issue 68: https://github.com/gaubert/gmvault/issues/68 .

The command to export is: gmvault export -d DB_DIR -t FORMAT OUTPUT_DIR

Currently valid formats are 'maildir' and 'mbox'. The maildir variant is 'Maildir++' as used by dovecote, and indeed the exported maildir can be used as-is as a dovecote backend. The original 'mboxo' variant of mbox is used, with '.sbd' as a suffix for directories as used by Thunderbird.

As mentioned in the issue, the maildir (or mbox) format duplicates emails, once per label. This is clearly not optimal, but there's no way to avoid it. Users should continue to use gmvault's internal format for everyday use, only exporting to maildir when they need it.