Use robfig/cron to parse cron lines

[rubiojr]

Handles parsing better, supporting the cron spec.

Partially addresses #293

[rubiojr]

This is ready for 👀 now. I ran go mod tidy so the diff is a bit larger than necessary.

[rubiojr]

The format supported is documented in https://godoc.org/github.com/robfig/cron.

Field name   | Mandatory? | Allowed values  | Allowed special characters
----------   | ---------- | --------------  | --------------------------
Minutes      | Yes        | 0-59            | * / , -
Hours        | Yes        | 0-23            | * / , -
Day of month | Yes        | 1-31            | * / , - ?
Month        | Yes        | 1-12 or JAN-DEC | * / , -
Day of week  | Yes        | 0-6 or SUN-SAT  | * / , - ?

Tested with an old config I had around and seems to be backwards compatible. It seems to fix some issues the current parser has also, like no support for */X periods.

[muesli]

Fantastic!