Add checkbox recipe

A tiny library for manipulating textual checkboxes in buffers. It has one main entry point (`checkbox-toggle`). The [README](https://github.com/camdez/checkbox.el/blob/master/README.md) is pretty extensive if you want to know more!

I wrote it and it lives on GH at [camdez/checkbox.el](https://github.com/camdez/checkbox.el).

Thanks!