support question mark

[yyscamper]

many of crontab implemenation support question mark on DayOfWeek and DayOfMonth field:

• https://docs.oracle.com/cd/E12058_01/doc/doc.1014/e12030/cron_expressions.htm
• https://en.wikipedia.org/wiki/Cron
• https://techdocs.broadcom.com/us/en/symantec-security-software/endpoint-security-and-management/cloud-workload-protection-for-storage/1-0/Scan_Configuration_7/about-special-characters-in-cron-expressions-v123769576-d4995e65374.html

if croniter supports question mark as well, it will be bettter to cover more cases.

[kiorky]

If you want such a feature, i'm not against, but you ll have to submit a PR for :-).

[kiorky]

released as 2.0.6

[yyscamper]

thanks for @JabberWocky-22