Add plugin option.

[vseventer]

This Pull Request:

Adds a plugins option.

Why is this PR needed?

Child compilers don't inherit plugins from the root compiler. Therefore, it is currently not possible to implement things like externals in the spawned files. Rather than adding support for all internal Webpack plugins one-by-one, why not allow a plugin option instead?

Example usage with defining an external React, which will be excluded from the bundle and instead contains a module simply exporting window.react:

file.js

import React from 'react';

webpack.config.js

{
  loader: 'spawn-loader',
  options: {
    plugins: [new webpack.ExternalsPlugin('var', { react: 'React' })]
  }
}

[erikdesjardins]

Released in 6.1.0, thanks!