Add package.json with minify script

It'd be nice to have minifications be consistent for any contributor. Here's how to use this:

1. `npm install` to get UglifyJS (done once)
2. `npm run minify` to put minified code in loadCSS.min.js (done every time you want to minify)

That's it!

We might want to update package.json to be publishable to npm, but it's private for now.