[Feature request] terraforming iamugm : aws_iam_user_group_membership

[rfrail3]

Hello,

It would be nice to have the option "iamugm" because, in Terraform, this option allow import and the similar one that Terraforming uses "iamgp" doesn't.

Upstream suggest the use of aws_iam_user_group_membership instead of aws_iam_group_membership due conficts.

https://www.terraform.io/docs/providers/aws/r/iam_group_membership.html

Thanks,