unpack(filename, cb, manifest) {
    pUnpack[platform](filename, cb, manifest, this.options.temporaryDirectory);
  }
Will unpack the `filename` in temporary folder.
For Windows, [unzip](https://www.mkssoftware.com/docs/man1/unzip.1.asp) is used (which is [not signed](https://github.com/edjafarov/node-webkit-updater/issues/68)).

@param {string} filename
@param {function} cb - Callback arguments: error, unpacked directory
@param {object} manifest