This change adds a `coco plan` command which is simply a shortcut
to the more verbose `coco deploy --dry-run`. This will make demos
flow nicer and elevates planning, an important activity, to a more
prominent position. The `--dry-run` (aka `-n`) flag is still there.
This change also renames `coco env destroy` to just `coco destroy`.
This is consistent with deploy and plan being at the top-level. We
now use `coco env` purely for evironment management commands (init,
config, rm, etc).
