feat: support for running immediately

[zeewell]

No description provided.

[mxAge18]

@robfig this feature is usefull, please supoort this, cheers

[ryjogo]

Agreed, this is a good feature to have.

[alaingilbert]

@zeewell @mxAge18 @ryjogo I made a fork that addresses almost all issues of this repo.
https://github.com/alaingilbert/cron

If you want to run a specific job at a random place in your code ->

c := cron.New()
id, _ := c.AddJob("* * * * *", func() {
	fmt.Println("hello world")
})
c.Start()
// ...
c.RunNow(id)

Or if you want to run the job when the cron starts ->

_, _ = c.AddJob("*/10 * * * * *", func() {
	fmt.Println("this job runs as soon as cron is started, then at every 10th seconds", time.Now())
}, cron.RunOnStart)