add boolean CRON env var to support running container as a cronjob

[robertlestak]

The current configuration will run the container forever, but the vast majority of that time is spent simply waiting for the next run. With this variable, the user can set CRON=1 and then schedule the docker container to run as a cronjob which will run once and exit on completion.

[0xFEEDC0DE64]

nice one! i applied a similar patch to my copy of the repo :)