Please add support of 'Wn' in the day of month field - nth weekday of the month

[foldvari]

Currently croniter does not accept CRON expressions like "0 9 1W * *" (first weekday of the month) which is a huge limitation in many real world use cases. This is how it is supposed to work:

W
The 'W' character is allowed for the day-of-month field. This character is used to specify the weekday (Monday-Friday) nearest the given day. As an example, if "15W" is specified as the value for the day-of-month field, the meaning is: "the nearest weekday to the 15th of the month." So, if the 15th is a Saturday, the trigger fires on Friday the 14th. If the 15th is a Sunday, the trigger fires on Monday the 16th. If the 15th is a Tuesday, then it fires on Tuesday the 15th. However, if "1W" is specified as the value for day-of-month, and the 1st is a Saturday, the trigger fires on Monday the 3rd, as it does not 'jump' over the boundary of a month's days. The 'W' character can be specified only when the day-of-month is a single day, not a range or list of days.

Source Wikipedia

It should accept nW and Wn as well.