code: function makeDefaultStep(step, cb) {
          if ('string' === typeof step) {
              step = {
                  id: step,
                  cb: makeDefaultCb(cb)
              };
          }
          step._defaultStep = true;
          return step;
      }
docs: #### makeDefaultStep
  
  Flags or create a step object marked as `default`
  
  @param {object|string} step The step object to mark. If a string
  is passed, a new step object with default cb is created.
  @ param {function} cb Optional A function to create the step cb
  
  @return {object} step the step flagged as `default`
  
  @see makeDefaultCb
  @see isDefaultStep