Provide method to specify Terraform version per module

At the moment, each module needs a constraint, but ideally in the root module, each sub-module and each example.

Maybe a configuration on the module (or specified in the terrareg.json file) that determines the verison of Terraform that should be used for extraction could be useful? Alternatively, could the entire code-base be scanned for any terraform version constraints and be used as a fallback?