Co-operate with formats that use tags of their own such as CloudFormation

CloudFormation has its own functions such as `!Subst` and `!Join` etc. In order to be able to input and output CloudFormation templates, Emrichen needs a way to co-operate with foreign tags.

(Bad) options:

0. Keep disallowing foreign tags and require CloudFormation users to use the `{ "Fn::Join": […] }` form of CFN functions instead of the tag-based `!Join` form
1. Prefix our own tags with some prefix, reducing the risk of collision, and pass foreign tags to the output as-is.
3. Require foreign tags to be prefixed and strip the prefix upon processing