Support Cognito groups

[jstarmx]

Related to https://github.com/rahulpsd18/cognito-backup-restore/issues/10

Adds the option to include the Cognito groups that a user is included in when backing up/restoring.

Other changes:

• Promisify the restoreUsers function so that awaiting it works correctly here: https://github.com/rahulpsd18/cognito-backup-restore/blob/master/src/cli/cli.ts#L42

[JoVidalis]

@rahulpsd18 Any movement on merging this in? It would be extremely useful to the community.

[rboboc]

Is this project still maintained? I'm almost sure that I'm not the only one who is waiting for this merge.