[notification] Construct intervals based off cron strings as seen in /etc/crontab

While this is highly technical in nature, it'd be nice if I could just pass a cron string to the plugin so that it schedules an interval notification the way I want it without me bothering much about how to interpret the DateMatch object... which took me a ton of time. But also, supporting a cron string should open up the road for us to schedule more complex intervals...

For example, if I want a notification that's scheduled at 00:00, 00:30, 12:00 and 12:30 every second day of month (so every 2 days in a month of 30 days), I could just want to write a cron string that looks like `0,30 00,12 */2 * *`.