Support custom styleLoaders for extract-text-webpack-plugin

[vjpr]

make sure your 'styleLoaders' array starts with 'style' or 'isomorphic-style' at index #0

I had to patch style-loaders and publish as @vjpr/style-loaders. This hard-coded requirement for style-loaders breaks my config.

I think this should be configurable in case someone wants to use a custom style-loader.

Would be good to have a option to disable this error.

[justin808]

I'd be supportive of a PR with additional documentation for this.