Custom group names

[tallen11]

Adds support for supplying a custom group name in case you don't want to use the default of group.<bundle_id>. I had an existing app that I couldn't change the bundle id or group name for and so I needed this functionality. You'd use it as follows

ReceiveSharingIntent.getReceivedFiles(
      files => {
        // do something
      },
      error => alert(error),
      'com.your.bundle.id',
      'group.your.custom.name',
);

I had to reformat all the files in order to commit to the repo, so sorry about the huge diff.