Adding checking whether cron ran in the last 480 minutes

[mxroo]

For this to replace the Drupal nagios module https://www.drupal.org/project/nagios for my coop, we need to also check cron. I copied the existing formatting and syntax for all the other checks, and added a -c option that would check the last time that cron ran as reported by drush vget --exact cron_last.