Add helm-sage.

[helm-sage](https://github.com/stakemori/helm-sage) provides a [helm](https://github.com/emacs-helm/helm) source for [sage-shell-mode](https://github.com/stakemori/sage-shell-mode).

I am the author and the maintainer of [helm-sage](https://github.com/stakemori/helm-sage).

Please merge this pull request.