Add top-level 'aliases' config to webpack config

[insin]

This is a common piece of config in my projects once they grow to any size:

  webpack: {
    extra: {
      resolve: {
        alias: {
          'img': path.resolve('src/img'),
          'src': path.resolve('src')
        }
      }
    }
  }

It would be handier to configure it as:

  webpack: {
    aliases: {
      'img': path.resolve('src/img'),
      'src': path.resolve('src')
    }
  }

(I've been tripped up in the past by forgetting the resolve object, too).