code: function extend (prototype, utils) {
    const { debug, strong } = utils.cnsl;
  
    /**
     * Compress file contents
     * @param {Object} buildOptions
     *  - {Boolean} bootstrap
     *  - {Boolean} boilerplate
     *  - {Boolean} browser
     *  - {Boolean} bundle
     *  - {Boolean} compress
     *  - {Array} ignoredFiles
     *  - {Boolean} helpers
     *  - {Boolean} watchOnly
     * @param {Function} fn(err)
     */
    prototype.compress = function compress (buildOptions, fn) {
      let content = this.content;
  
      const asString = ('string' == typeof content);
  
      // Requires buffer
      if (asString) content = new Buffer(content);
  
      imagemin
        .buffer(content, {
          plugins: [
            gifsicle(),
            jpegtran(),
            optipng({ optimizationLevel: 3 }),
            svgo()
          ]
        })
        .then((content) => {
          debug(`compress: ${strong(this.relpath)}`, 4);
          if (asString) content = content.toString();
          this.setContent(content);
          fn();
        })
        .catch(fn);
    };
  }
docs: Extend 'prototype' with new behaviour
  @param {Object} prototype
  @param {Object} utils