Add Options() accessor on Task for external option merging

Exposes the opts field so callers can read a task's options to merge
with additional per-task options at batch time.

asynq.go

@@ -40,6 +40,7 @@ type Task struct {
 func (t *Task) Type() string               { return t.typename }
 func (t *Task) Payload() []byte            { return t.payload }
 func (t *Task) Headers() map[string]string { return t.headers }
+func (t *Task) Options() []Option          { return t.opts }
 
 // ResultWriter returns a pointer to the ResultWriter associated with the task.
 //