code: function postProcess (output) {
    var self = this;
    this.rules.forEach(function (rule) {
      if (typeof rule.append === 'function') {
        output = join(output, rule.append(self.options));
      }
    });
  
    return output.replace(/^[\t\r\n]+/, '').replace(/[\t\r\n\s]+$/, '')
  }
docs: Appends strings as each rule requires and trims the output
  @private
  @param {String} output The conversion output
  @returns A trimmed version of the ouput
  @type String