Support outputting pyplate as yaml

[tehsmyers]

CFN now supports YAML output. It should be pretty simple to add a mechanism to output in this YAML format rather than JSON: https://aws.amazon.com/blogs/aws/aws-cloudformation-update-yaml-cross-stack-references-simplified-substitution/

Care needs to be taken with "!function" references, and support needs to be added for !ImportValue and !Sub

[scottlinux]

FWIW for anyone else looking for yaml love in the meantime, one can convert / validate existing json templates to yaml with aws-cfn-template-flip:

https://github.com/awslabs/aws-cfn-template-flip