Explain crons in crontab

An `--explain` flag would be brilliant. It would list out the cron jobs you have in the crontab file and explain when they will run.

Example output:

```
0 0 * * FRI /backup		At 12:00 AM, only on Friday
0 * * * * /clean-cache		Every hour
05 01 * * SUN#1			At 01:05 AM, on the first Sunday of the month
```