Add ConfigGenerator

[Kocal]

A bit related to #634

Add a config generator, aside the old plain-object config? We would have access to typings and a easier usage.

const { ConfigGenerator } = require('@yproximite/yprox-cli');

const configGenerator = ConfigGenerator.init()
  .configureAssets('app', (asset) => {
    asset.addRollupEntry('src/yprox-cli.config.js/my-file.js', rollupOptions => {});
    asset.copyFiles(from, to);
    asset.optimizeImages(from, to);
    asset.addSassEntry(...); 
    // ...
  })
  .configureRollup(options => {
     // ...
   })
;

module.exports = configGenerator.generate()

Methods to implement:

• .addEntry(name, ['...'], {})
• .addStyleEntry(name, ['...'], {}),
• .copyFiles(from, to)
• .optimizeImages(from, to)
• .configureRollup(rollup => { })
• .configureESLint(eslint => { })
• .configurePostCSS(postcss, => {})
• .configureAutoprefixer(autoprefixer => {});
• .configureImagesOptimization(options => {});