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
if tis is not None:
        states_by_task_id = {ti.task_id: ti.state for ti in tis}
    for task in dag.tasks:
        node_attrs = {
            "shape": "rectangle",
            "style": "filled,rounded",
        }
        if states_by_task_id is None:
            node_attrs.update(
                {
                    "color": _refine_color(task.ui_fgcolor),
                    "fillcolor": _refine_color(task.ui_color),
                }
            )
        else:
            state = states_by_task_id.get(task.task_id, State.NONE)
            node_attrs.update(
                {
                    "color": State.color_fg(state),
                    "fillcolor": State.color(state),
                }
            )
        dot.node(
            task.task_id,
            _attributes=node_attrs,
        )
        for downstream_task_id in task.downstream_task_ids:
            dot.edge(task.task_id, downstream_task_id)
    return dot