add time formatting options to cron describe

[illegalprime]

From #54 This is a naive solution, but I realized that a time zone offset might shift the days, and even the month! Since days and month are handled separately this makes writing a correct implementation more complicated. I'm too busy to give it a shot anytime soon. :(

[MichaelXavier]

Maybe it would be best to keep this open until there's more time. It feels weird to ship it if it's incorrect.