Add webpack 3.0 to acceptable peerDependencies

[fuller]

I'm currently getting peer dependency warnings about this package when trying to use Webpack 3.0.

warning "extract-text-webpack-plugin@2.1.2" has incorrect peer dependency "webpack@^2.2.0".

Everything appears to be working as expected, so it should be safe to add 3.0 to the acceptable peerDependencies.

[jsf-clabot]

All committers have signed the CLA.

[michael-ciniawsky]

WIP in #540 && #553 Thx :)