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
#
#
#
"""Renderer DAG (tasks and dependencies) to the graphviz object."""
from typing import List, Optional
import graphviz
from airflow.models import TaskInstance
from airflow.models.dag import DAG
from airflow.utils.state import State
def _refine_color(color: str):
    """
    Converts color in #RGB (12 bits) format to #RRGGBB (32 bits), if it possible.
    Otherwise, it returns the original value. Graphviz does not support colors in #RGB format.
    :param color: Text representation of color
    :return: Refined representation of color
    """
    if len(color) == 4 and color[0] == "#":
        color_r = color[1]
        color_g = color[2]
        color_b = color[3]
        return "#" + color_r + color_r + color_g + color_g + color_b + color_b
    return color
def render_dag(dag: DAG, tis: Optional[List[TaskInstance]] = None) -> graphviz.Digraph: