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]
function _reduceStream (stream$, reducer, cb) {

	stream$
		.pipe(reduce(reducer.fn, reducer.memo))
		.once('data', function (reduction) {
			cb(null, reduction);
		})
		.once('error', cb);
}