feat: support job run immediately option

Author: fufuok

cron.go

@@ -190,6 +190,8 @@ func (c *Cron) Schedule(schedule Schedule, cmd Job, entryOpts ...EntryOption) En
 	return entry.ID
 }
 
+var LongLongAgo = time.Unix(0, 1)
+
 // EntryOption is a hook which allows the Entry to be altered before being
 // committed internally.
 type EntryOption func(*Entry)
@@ -202,6 +204,11 @@ func WithPrev(prev time.Time) EntryOption {
 	}
 }
 
+// WithRunImmediately this job will be run as soon as the job is added
+func WithRunImmediately() EntryOption {
+	return WithPrev(LongLongAgo)
+}
+
 // Entries returns a snapshot of the cron entries.
 func (c *Cron) Entries() []Entry {
 	c.runningMu.Lock()

cron_test.go

@@ -687,9 +687,10 @@ func TestJobWithCustomPrev(t *testing.T) {
 	// expected timeline: 1s ... 4s ... stop (2 calls)
 	// if prev was ignored, the func would only be called once (at 3s)
 	cron.AddFunc("@every 3s", func() { atomic.AddInt64(&calls, 1) }, WithPrev(time.Now().Add(-2*time.Second)))
+	cron.AddFunc("@every 4s", func() { atomic.AddInt64(&calls, 1) }, WithRunImmediately())
 	cron.Start()
 	time.Sleep(5 * time.Second)
-	if atomic.LoadInt64(&calls) != 2 {
+	if atomic.LoadInt64(&calls) != 4 {
 		t.Errorf("called %d times, expected 2\n", calls)
 	}
 }