Add flag so we can run in one-shot mode from cron

[mvanbaak]

Personally, I prefer to schedule this script with cron, so it runs every 10 minutes.
The while True is nice and all, but if something goes wrong the script terminates.
One can of course 'fix' this by running it with supervisord or systemd. But cron is also an option.

This patch adds the command line option ```--cron``
When used, the script will do one run of torrent info collection, send it to influxdb, and exit.