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
airflow/utils/dot_renderer.py
"""
    Renders the DAG object to the DOT object.
    If an task instance list is passed, the nodes will be painted according to task statuses.
    :param dag: DAG that will be rendered.
    :type dag: airflow.models.dag.DAG
    :param tis: List of task instances
    :type tis: Optional[List[TaskInstance]]
    :return: Graphviz object
    :rtype: graphviz.Digraph
    """
    dot = graphviz.Digraph(
        dag.dag_id,
        graph_attr={
            "rankdir": dag.orientation if dag.orientation else "LR",
            "labelloc": "t",
            "label": dag.dag_id,
        },
    )
    states_by_task_id = None