Allow sealed-secrets to be installed as a kustomize `base/resource`

With [kustomize](https://github.com/kubernetes-sigs/kustomize/blob/master/docs/glossary.md#base) allowing composition of multiple `kustomize` layers it would be great to have **sealed-secrets** to be able to be installed in such way.

Here is what it would look like from someone wanting to install sealed-secrets in their cluster:

```yaml
# kustomization.yaml
bases:
- github.com/bitnami-labs/sealed-secrets//path/to/kustomize/dir?ref={git tag}
```

That is a very handy way of installing the project as well as keeping up with new releases. It would only require as part of the release process to have a folder that has a `kustomization.yaml` file referencing all necessary resources.

A few projects have gone that route and it has streamlined things for my team... The latest example that we saw of that was [argocd](https://argoproj.github.io/argo-cd/operator-manual/declarative-setup/#manage-argo-cd-using-argo-cd)