Allow style-loader to accept loaders

[timkelty]

This adds a loaders option to style-loader, in which users may pass their own custom css loaders (postcss, sass, etc)

Why?

• CSS loaders can be tricky to configure correctly, especially when factoring in extract-text, css modules, hot-reloading, and css-loader's importLoaders option.
• Prevents the need for community-sourced middlewares that do nothing more than add a loader to the style rule (e.g. https://github.com/barraponto/neutrino-middleware-postcss, https://github.com/kopacki/neutrino-dev/blob/master/packages/neutrino-preset-sass/src/index.js)

Example usage

module.exports = {
  use: [
    [
      '@neutrinojs/web',
      {
        style: {
          loaders: [

            // No options, auto-generated useId (css-*)
            require.resolve('postcss-loader'),

            // With options, specified useId
            {
              useId: 'postcss',
              loader: require.resolve('postcss-loader'),
              options: {}
            }
          ]
        }
      }
    ],
  ]
};

• The result is the loaders added in order: style, css, sass-loader, postcss-loader.
• css-loaders's importLoaders has been set properly, taking into account the # loaders your have, and options.modules.
• Everything still works with extract-text, and HMR :)

Related: #20

[eliperelman]

First, let me say that this is sooooo what I've been wanting. This is exactly the direction this middleware needs to go.

My only concern is making sure we have appropriate docs for this. Otherwise, this is great! 😍

[eliperelman]

Let's mess with the formatting a little here:

const loaders = [
  {
    ...
  },
  {
    ...
  },
  ...options.loaders
]
.map(() => {
  ...
});

[eliperelman]

I like this, but this will need to be clearly noted in the documentation, that without the useId, it will generate an id in this format.

[eliperelman]

I already have the docs written in #574.