Feature Request: Use Lookups in Post-Build Hooks

I have integrated stacker into a few projects now and have run into the same issue a few times where I want to run a post build hook and need to access the stack outputs. Currently, unless Im doing something wrong, it seems that I can't use the the `${output stackName::outputName}` lookup or any other lookup in a post build hook.

I have had to do resort to using boto3 directly in my hook code which I really don't like.