In my configuration, I want to dynamically lookup whether a stack is enabled, like... ``` my-stack: enabled: ${custom something} template_path: stacker/templates/my-template.yml.j2 ``` where my [lookup](https://stacker.readthedocs.io/en/latest/lookups.html#custom-lookup) (in this case `custom`) resolves to _true_ or _false_, thus toggling stack creation.
