add --dashboard & --hook options to supervisor mode

[noblem]

During the SOP workshop there was some discussion about the mechanism by which gdac_cron can be used for folks outside of the GDAC Unix group, proper. Since the main purpose of the cron jobs is dashboarding, maybe this could be handled in a different way, for example by having the dashboard updated after every timeout/loop of supervisor? And initiated with --dashboard option?

A generalized way to do this would be to give the supervisor the means of running arbitrary code after each iteration (or after some regular time interval), via some kind of --hook option or something, which specifies how a module or function or script to invoke per iteration of the supervisor loop (or, again, after every N seconds, or something)