Feature Request: Add ability to substitute using Rain::Embed and Rain::Include

[cmckni3]

I need to substitute some variables in an external file and embed/include the content in the CloudFormation template.

Current Use Cases

• Substitute CloudFormation pseudoparameters, such as AWS::Region and AWS::AccountId in external JSON IAM policy documents.
• Substitute the domain name in the HTML email template

What I tried that doesn't work

nil error

...
PolicyDocument: !Sub "${Rain::Include my-iam-policy.json}"
...

Invalid YAML

...
PolicyDocument: !Sub !Rain::Include my-iam-policy.json
...

Fn::Sub is removed

PolicyDocument: !Sub |
  Rain::Include my-iam-policy.json

My current workaround is to continue using Fn::Sub and/or Fn::Join for the portion I need to be dynamic. I preprocess the content, and rain embeds the preprocessed file into the email template.