Add a recipe for sage-shell-mode.

[sage-shell-mode](https://github.com/stakemori/sage-shell-mode) provides a front end for [Sage](http://www.sagemath.org/) (a free mathematical software).

I am the author and the maintainer of [sage-shell-mode](https://github.com/stakemori/sage-shell-mode).

Please merge this pull request.