code: function(convert, options) {
        var callback = options && 'process' in options ? options.process : options;
  
        if(typeof(callback) !== 'function') {
          return convert;
        }
  
        return function(string, options) {
          return callback(string, convert, options);
        }
      }
docs: Sets up function which handles processing keys allowing the convert function to be modified by a callback