Webpack loader for static extraction from separate files

Me @markdalgleish and @lttb in separate chats discussed that JSS has no examples on static extraction from separate files, even though it can be easily done and big projects like https://www.kijiji.ca/ do it that way.

@markdalgleish has written this [webpack loader](https://github.com/markdalgleish/jss-loader) long time ago.

## Todo

- [ ] update this plugin with the latest JSS and webpack versions.
- [ ] create an example that also uses autoprefixer.
- [ ] make it use default preset
- [ ] allow passing custom jss from the config
- [ ] link that project and example from the main readme.md