Airflow-clickhouse-plugin

* Airflow version tested: 2.2.0.
* Note on `pandas` dependency [added](https://github.com/whisklabs/airflow-clickhouse-plugin#note-on-pandas-dependency). `pandas` is an extra dependency now.

Available on PyPI: https://pypi.org/project/airflow-clickhouse-plugin/0.8.0/

* Airflow versions tested: 2.0.0, 2.0.1, 2.1.0.
* ClickHouse driver version tested: 0.2.0.

Available on PyPI: https://pypi.org/project/airflow-clickhouse-plugin/0.7.0/

* Airflow v2.0.1 support is added.
* Airflow v1 support is dropped. Use plugin versions below 0.6.0 for compatibility with Airflow~=1.10.6.
* `tests.*` modules are completely excluded from distribution package.

Available on PyPI: https://pypi.org/project/airflow-clickhouse-plugin/0.6.0/

Added support for arbitrary `clickhouse_driver.Client` [arguments](https://clickhouse-driver.readthedocs.io/en/latest/api.html#clickhouse_driver.Client) via Airflow's Connection's `extra` property.