Add support for Parameter Groups

[screwdhila]

Newbie in Go. So far this package was very useful in the project I'm working. I'm using this to generate CFN template. I noticed one feature is missing that is support for Parameter Groups using AWS::CloudFormation::Interface which is used by the AWS console to display relevant parameters in groups.

This requires Template struct to include Metadata field.

I see the schema.go includes various type (CloudFormationInterface, ParameterGroupsList etc etc) as resource types and wasn't sure how to use them to implement the Metadata field in Template struct.

I have come up with an alternate approach (patch attached) which works but probably not a good approach (again I'm newbie and in very early stage of my learning GO). Appreciate some suggestion and guidance.
metadata.txt