code: function _reduceStream (stream$, reducer, cb) {
  
  	stream$
  		.pipe(reduce(reducer.fn, reducer.memo))
  		.once('data', function (reduction) {
  			cb(null, reduction);
  		})
  		.once('error', cb);
  }
docs: Reduce a stream of data into a single value.
  
  @param  {[type]}   stream$ [description]
  @param  {Function} reducer [reducer object]
  @param  {Function} cb      [description]
  @return {[type]}           [description]