Upgrade isomorphic-style-loader to React 16.6.0 and use new Context API

[mglace]

I want to take advantage of the new React.Component.contextType property in my React application but it requires React 16.6.0. I'm not able to upgrade because of a dependency on isomorphic-style-loader which still uses the legacy context API. Here is a proposed change on how the isomorphic-style-loader can leverage the new React context API.

Usage:

import StyleContext from 'isomorphic-style-loader/lib/styleContext';

class App extends React.PureComponent {
  static propTypes = {
    children: PropTypes.element.isRequired,
  };

  render() {
    const css = new Set();
    const insertCss = (...styles) => styles.forEach(style => css.add(style._getCss()));

    return (
      <StyleContext.Provider value={{ insertCss }}>
        {React.Children.only(this.props.children)}
      </StyleContext.Provider>
    );
  }
}

[coveralls]

Coverage increased (+0.6%) to 84.211% when pulling eb6d190 on mglace:master into 0c49766 on kriasoft:master.

[Dakkers]

@frenzzy could we get this merged, and drop support for Node 4, 5 in Travis? they've been past their end-of-life for quite some time, and even Node 6 is reaching its end of life soon.

[piglovesyou]

I made a one before but this is better.

@koistya @frenzzy Because of silence like this thread, React Starter Kit is getting old. I do like RSK. Let's take some action before it's too late :)

[piglovesyou]

@koistya @frenzzy Please find someone who can merge this PR if you don't have time, please.

[frenzzy]

Thank you!

[piglovesyou]

@frenzzy Thank you! Can I ask when do you plan to release it?

[frenzzy]

Released as v5.0.0 🙏 🎉

[mglace]

Thank you!