code: function getCriticalDestination(rule, dest) {
    rule.walkDecls('critical-filename', function (decl) {
      dest = decl.value.replace(/['"]*/g, '');
      decl.remove();
    });
    return dest;
  }
docs: Identify critical CSS destinations.
  
  @param {object} rule PostCSS rule.
  @param {string} Default output CSS file name.
  @return {string} String corresponding to output destination.