Indicate primary CSS file in package.json

Many modules in npm are starting to expose their css entry files in their package.json files. This allows tools like `npm-css`, `rework-npm`, and `npm-less` to import a module's styles from the node_modules directory. Bootstrap is [in the process](https://github.com/twbs/bootstrap/pull/12441) of adopting this standard as well.
Merge pull request #2432 from bclinkinbeard/patch-1

Indicate primary CSS file in package.json