provide ant way to change the value of Now for testing purposes

[aaron-sysdig]

in order to test code which uses this library it would be very helpful if this library did not directly call time.Now() but instead allowed the user to specify how this library determines when Now is

[jiangzhongqiang]

in order to test code which uses this library it would be very helpful if this library did not directly call time.Now() but instead allowed the user to specify how this library determines when Now is

This is my optimized version that can meet your needs（ https://github.com/jiangzhongqiang/cron/ ）
I added a new option Clock
usage: cron.New(cron.WithClock(xxx))