Add lit-mode recipe.

This is a [new major mode](https://github.com/HectorAE/lit-mode) used for editing [`lit`](https://github.com/cdosborn/lit) template/macro files. `lit` is a literate programming tool, similar to noweb, written in Haskell. Its syntax is different, however, so I made a basic syntax highlighting mode that should work with polymode.

I am the original author and maintainer. I tested the recipe and installation and it works fine. Thank you for reviewing this. By the way, thank you for your continued work on MELPA; it's fantastic and I use it all the time.