Allow use parameters of type List<> in resources with properties where PrimitiveItemType = String and Type = List

[xrn]

Issue #, if available:
#306 #282

Description of changes:

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[xrn]

Like I mentioned in Issue - if we have PrimitiveItemType = String and Type = List - we should be able to use Parameters with type this means Ref::ParameterX (it is string) as well as []string{"A","B","C"}

Example

"AWS::RDS::DBCluster": {
      ...
      "VpcSecurityGroupIds": {
          "Documentation": "http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbcluster.html#cfn-rds-dbcluster-vpcsecuritygroupids",
          "DuplicatesAllowed": false,
          "PrimitiveItemType": "String",
          "Required": false,
          "Type": "List",
          "UpdateType": "Mutable"
        }

@PaulMaddox will be awesome if you can take a look

[PaulMaddox]

I think there's a pretty big downside to this approach - the API of goformation becomes less typed, more error prone for consumers, and harder to use. It shifts validation from compile time to runtime which is not a good thing. The more places we use interface{}, the less valuable this library is.

I think there is an opportunity to do something better here, and although it's a much bigger change, it fixes a lot of current issues with goformation...

One of the current issues with this library is that all structs accept properties by value. We need to move the libary to use pointers instead, which will fix issues such as #294, #181, #61 etc (there's more similar issues).

I think this would be the right opportunity to also include some smarter types that could allow the API of this library to retain it's strong typing, while allowing scenarios such as this issue.