Proof of concept: Plugin System

Created by: dceddia

(This is related to issue #670 (closed))

This commit adds a "plugins" module and integrates it into the dev and prod build scripts.

The design and functionality is intentionally minimal, but it allows a user to write a plugin or series of plugins to manipulate the Webpack config as necessary. The example given in the plugins.js file shows a plugin that adds a SASS loader:

module.exports = function(webpackConfig) {
    webpackConfig.module.loaders.push({
      test: /\.scss$/,
      loaders: ["style", "css", "sass"]
    });
    return webpackConfig;
};

But conceivably any sort of Webpack config transformation could be done here.

Plugins are loaded and run, in order, from the plugins key in package.json. This would load a module named cra-plugin-sass, which would need to be npm install'd separately.

  "plugins": [
    "cra-plugin-sass"
  ]

We could also explore auto-discovery of plugins ala Babel or Karma, by searching for packages matching the "cra-plugin-*" prefix. However, the loading order does matter (since the config is transformed by every plugin in turn) and an array of plugins is more explicit.