add a recipe for imgix

Below is the requested info for adding a recipe.

* This is a major mode for editing images in emacs via [imgix](http://www.imgix.com)
* Direct link to package code: https://github.com/imgix/imgix-emacs
* I wrote the imgix emacs code and work for imgix.

Please let me know if there's anything I need to do. FWIW, I successfully installed via the sandbox method mentioned in your README. Thanks!