cron.WithSeconds: add special constructor for the cron spec variety with seconds

README.md

@@ -25,6 +25,10 @@ Updates required:
   UPDATING: To retain the old behavior, construct your Cron with a custom
   parser:
 
+      // Seconds field, required
+      cron.New(cron.WithSeconds())
+
+      // Seconds field, optional
       cron.New(
           cron.WithParser(
               cron.SecondOptional | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor))

option.go

@@ -15,6 +15,14 @@ func WithLocation(loc *time.Location) Option {
 	}
 }
 
+// WithSeconds overrides the parser used for interpreting job schedules to
+// include a seconds field as the first one.
+func WithSeconds() Option {
+	return WithParser(NewParser(
+		Second | Minute | Hour | Dom | Month | Dow | Descriptor,
+	))
+}
+
 // WithParser overrides the parser used for interpreting job schedules.
 func WithParser(p Parser) Option {
 	return func(c *Cron) {