Allow lookup in stack `enabled` field

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.