code: function streamify (opts, transform) {
    return through2.obj(
      // (vfile: VinylFile, enc: String, cb: Function)
      (vfile, enc, cb) => {
        var contents = vfile.contents.toString(enc);
        try {
          contents = transform({
            contents: contents,
            fetcher: opts.fetcher,
            filePath: vfile.path,
            index: opts.index
          });
        } catch (err) {
          cb(err);
          return;
        }
        vfile.contents = new Buffer(contents, enc);
        cb(null, vfile);
      }
    );
  }
docs: streamify ({ index: FetcherIndex }, transform: Function) => Stream