Conditionally import cloud hooks
With the current imports, users are required to have modules installed for both aws and gcp so that S3Hook and GoogleCloudStorageHook may be imported. However, it's not likely a user will need to have both installed, so it is better to delay and import them depending on the value of destination.
Also updates readme with the current operator doc string.
@dankolbman was the hook for gcp imported fine for you ? I'm having issues: https://github.com/airflow-plugins/github_plugin/pull/5
@louisguitton I removed the gcp hook from the import here since I only use the S3 hook: https://github.com/airflow-plugins/github_plugin/blob/41d3d2a175a54f83889cc9561239df6e8ce9edc8/operators/github_to_cloud_storage_operator.py#L8