function sourceMaps(devtool = 'cheap-module-eval-source-map') {
  return (context, util) => {
    context.sourceMaps = true

    return util.merge({ devtool })
  }
}
Just a convenience wrapper to enable sourcemaps in an easier-to-read fashion
than `setDevTool()`.
@TODO: Only sets the javascript sourcemaps now. Would be nice to make loaders
       enable their specific sourcemaps when `sourceMaps()` is used.

@param {string} [devtool]
@return {Function}