#15 Add an option to avoid archiving DRAFT mail

[Fabio Muzzi]

It should be useful to have an option to not archive
mails that have DRAFT status set.

It should work exactly like the "--include-flagged",
being maybe "--include-drafts". In maildirs, drafts
have a "D" after the ";2," string. In mailboxes I don't
know if this flag exists or not.