Gusty

Revert the adding of the `macros` module to `airflow_default_macros`.

- Using `jupyter/r-notebook` as a base image to decrease build time.

- General updating of `requirements.txt`.

- `render_on_create` is disabled by default, to better align with Airflow's default behavior.

- Airflow's `user_defined_macros` and gusty's `dag_constructors` are consolidated while building the DAG, with priority always given to Airflow's `user_defined_macros`. This ensures anything user-provided is available both as YAML constructors (`!`) in gusty and in Jinja context (`{{}}`) in Airflow.

render_on_create` argument available in `create_dag` functions. Set `render_on_create=False` to disable gusty's default behavior of rendering Jinja at the time of DAG creation.

extra_tags` argument in `create_dag` allows users to specify additional tags to append to any existing DAG tags.

Removes unneeded `airflow` imports. Should be compatible as far back as `1.10.10`.

Thank you to josh-fell for the contribution! :tada: