Hello,

Airflow ability to [display DAG in CLI](http://apache-airflow-docs.s3-website.eu-central-1.amazonaws.com/docs/apache-airflow/latest/usage-cli.html#display-dags-structure) with command `airflow dags show`, but unfortunately this command does not display Task Groups. It would be great if the Task Groups were correctly marked in the diagrams.

<img width="1268" alt="Screenshot 2020-12-14 at 02 28 58" src="https://user-images.githubusercontent.com/12058428/102030893-9f3e4d00-3db4-11eb-8c2d-f33e38d01997.png">
<img width="681" alt="Screenshot 2020-12-14 at 02 29 16" src="https://user-images.githubusercontent.com/12058428/102030898-a2d1d400-3db4-11eb-9b31-0cde70fea675.png">

Best regards,
Kamil Breguła
tests/utils/test_dot_renderer.py
def test_should_render_dag(self):
        dag = DAG(dag_id="DAG_ID")
        task_1 = BashOperator(dag=dag, start_date=START_DATE, task_id="first", bash_command="echo 1")
        task_2 = BashOperator(dag=dag, start_date=START_DATE, task_id="second", bash_command="echo 1")
        task_3 = PythonOperator(
            dag=dag, start_date=START_DATE, task_id="third", python_callable=mock.MagicMock()
        )
        task_1 >> task_2
        task_1 >> task_3
        dot = dot_renderer.render_dag(dag)
        source = dot.source
        
        assert "label=DAG_ID" in source
        assert "first" in source
        assert "second" in source
        assert "third" in source
        assert "first -> second" in source
        assert "first -> third" in source
        assert 'fillcolor="#f0ede4"' in source
        assert 'fillcolor="#f0ede4"' in source
    def test_should_render_dag_with_task_instances(self):