Support helm plugin install command

[TimJones]

It would be very helpful if the registry plugin supported helm plugin install rather than having to curl ... | tar ....

In addition, the instructions for installation in the README for this repo are incorrect. The URL in the README is https://github.com/cn-app-registry/appr-cli/releases/download/v0.3.7-dev/registry-appr-v0.3.7-dev-linux-x64-helm-plugin.tar.gz, but the correct one (found on (this CoreOS blog post)[https://coreos.com/blog/quay-application-registry-for-kubernetes.html#using-helm-to-deploy-an-application]) is https://github.com/app-registry/helm-plugin/releases/download/v0.3.7/registry-helm-plugin-v0.3.7-dev-linux-x64.tar.gz.

Properly supporting the standard helm plugin install would avoid the confusion created.

[marians]

Seems like the entire install part in the README is misleading. I recommend going to https://github.com/app-registry/appr-helm-plugin#install for up-to-date info.