Upgrade style-loader to v0.21.0 to enable HMR for css modules

[holtat]

I've upgraded style-loader to address issue #144 where using CSS modules would not allow for HMR.

Here is the PR that was merged to allow for CSS modules to work with HMR on style-loader for reference: webpack-contrib/style-loader#298

I don't know of any breaking changes this update would cause.

[holtat]

It looks like this package is out of date on the master branch of create-react-app as well, but up to date on the next branch.

I'm not sure what's preferred. Should I try to update this fork with the next branch of create-react-app?