make stylus optional

[weisjohn]

The Readme.md says

There are currently 3 mandatory script files:

script.ejs - This is the HTML template for the layout of the descktop wallpaper
script.js - This is the Server-side(!!) script to be ran when the script is requested. This runs in your NodeJS instance so you can configure it to do whatever you want that is supported by NodeJS
script.styl- This is the Stylus CSS source file that will generate a script.css file when needed

and I don't want to force people to use stylus.