taskflow name is not setted

Currently taskflow name is not setted but  some user probably use the taskflow name in some situation

```go
// NewTaskFlow returns a taskflow struct
func NewTaskFlow(name string) *TaskFlow {
	return &TaskFlow{
		graph: newGraph(name),
	}
}

func (tf *TaskFlow) Name() string {
	return tf.name
}
```

So i will make a PR fow this issue