Add support for profile provider argument

[ctffarley]

When using the AWS provider for terraform, it is possible to specify the AWS CLI profile that is used within the AWS provider block like so:

provider "aws" {
  profile = "my_profile"
}

Similar to this request for role ARNs, I think it would be a good enhancement to implement the same argument either on the provider or file level for the SOPS provider.

[pkoch]

Here's how you can do it: #89 (comment)