code: function pipeToCb(stream, cb) {
      var reader = stream.getReader();
      return reader.read()
          .then(function pump(res) {
              if (res.done) {
                  return;
              }
              cb(res.value);
              return reader.read()
                  .then(pump);
          });
  }
docs: MixMaster: Mix & transform streams, and return a merged ReadableStream.
  
  TODO:
  - Hook up real ReadableStream
  - Tests
  elematch & mustachio to expose:
  makeTransform(options) -> transform(chunk, enqueueInReadableCb, transformDoneCb)
  streams standard:
  new TransformStream(transform) -> { writable, readable }
  WritableStream not yet available, so provide:
  mixmaster.pipeThroughTransform(input, transform) -> ReadableStream
  equivalent to: input.pipeThrough(new TransformStream(transform))
  alternative: polyfill pipeTo and pipeThrough