Taking ~40s to synchronize a ~75K message IMAP folder is
still a lot of time, so support an option to only touch
new messages.
This is similar to "offlineimap -q" (quick) or "mbsync --new"
switches, but lei already accepts "-q" as a shortcut for
--quiet. "--new" could work, but "--new-only" might be more
descriptive (or "--only-new"?), since the default fetches
also fetches new messages.
v2: warn for non-IMAP sources, I'm not sure it's worth it for
Maildir or other sources, yet. It will also make sense
for MH and JMAP once we support them.