It'd be great if I could flag certain folders such that offlineimap
only synchronises them e.g. once a day, once a week, or every Nth
run. It should not matter whether offlineimap -o is invoked by cron,
or whether the same process sleeps between runs.

I tried to implement this with state files and python magic in the
helper file, but it got very very messy and fragile. Since
offlineimap keeps metadata about each folder anyway, it'll probably
be fairly easy to implement this, once we've agreed on a config
representation.

Maybe a dictionary would be best, mapping folder name to
a frequency representation, e.g.:

  *5                            -> every fifth run
  daily,hourly,weekly,monthly   -> ibid.
  sundays,wednesdays,...        -> ibid.
  1st,3rd,16th,...              -> day of each month

Cheers,