code: function evaluatePackage(filename, attr, format) {
      var pkgs = require(path.resolve(filename)).pkgs;
      var pkg = pkgs[attr]();
      var expr = nijs.jsToNix(pkg, format);
      return expr;
  }
docs: Imports the given package composition CommonJS module and evaluates the
  specified package.
  
  @param {String} filename Path to the package composition CommonJS module
  @param {String} attr Name of the package to evaluate
  @param {Boolean} format Indicates whether to nicely format to expression (i.e. generating whitespaces) or not
  @return {String} A string containing the generated Nix expression