Run cron once a week / a month

[okrutny]

Currently it's not possible to run cron on a specified date and hour every month, i.e. issuing invoices 15th of a month. It would be useful, if one could specify "run cron every 15th of a month at 3 am". I could be provided in a similar way to RUN_AT_TIMES, for example RUN_AT_DAYS_OF_MONTH=['15:03:00'], where first part of string is a day of a month, second part is an hour and last part is minutes.

Going further, it would be also useful, if one could specify "run cron every monday at 3 am". In that case RUN_AT_DAYS_OF_WEEK could be used with strings like 'Mon:03:00', where first part is a day of a week, the rest is hour and minutes.

To provide more flexibility, also management command running specified cron class could be added, so it would be possible to specify in crontab to run that cron at that time without use of runcrons management command.

[chachra]

You can now run with specific cron names in the management command call.