Supporting a corner case, last day of a month

[yangyud-cn]

It would be great if we could expand the cron expression to support one corner case, last day of a month.

https://stackoverflow.com/questions/6139189/cron-job-to-run-on-the-last-day-of-the-month

Also, instead of using "L" for it, an alternative is to use "0" for this special day, such that it works easier with the existing integer valued parser.