Make `style-loader` not crash in node

**Warning: untested**

I think this'll work but I'm not sure if there's some test suite I should run or something.

The real solution for server-side rendering to prevent FOUC is to accumulate these into the head of the HTML response, but I think that's likely outside the scope of this loader. For now, I think we can get away with not crashing in Node. It'll at least make demos look good.

Does this belong in `style-loader` or should there be a new `isomorphic-style-loader`?