Swap from `cron` to `croner-rust` - Support wider range of cron features

[MGough]

The cron library is quite a strict implementation, which makes some things like 'run on the last day of each month' or 'run on the 3rd wednesday of each month' tricky, amongst others.

Croner seems to have wider support, so allowing it to be used instead of cron would be great. Perhaps it might even be possible to select which cron library to use?

I'm happy to try to help out with a move to Croner if it's likely to be merged rather than sit as an open PR, just let me know!