Support for Intervals extension

[Spikhalskiy]

Some cron parsers do support intervals like @every 1h30m10s

For example:
https://pkg.go.dev/github.com/robfig/cron#hdr-Intervals
https://github.com/robfig/cron/blob/master/doc.go#L26

Would be great to get them in cron-utils too

[jmrozanec]

@Spikhalskiy thanks! 😄