Move the complicated command tools out of the base release

As https://github.com/moooofly/harborctl/issues/5#issuecomment-449804308

> Suggest move the complicated commands like `statistic` and `tag retention policy` out of the base release, those features can be supported as plugins of CLI in future. Issues should be created for tracking.

related to https://github.com/moooofly/harborctl/issues/12