Add shim option for packages that don't define a style field or use index.css

This adds the ability to provide a `shim` config option for packages that neither specify a `style` field in their package.json nor provide their styles as `index.css`. The shim option is expected to be a hash, where the key is the package name and the value is the path to the CSS file. The only caveat is that the shimmed package must have a `package.json` file, but that seems like a safe assumption.

I added a test to verify the behavior, and if you're good with the change I can also update the README prior to merging.