Templating language for lookups

Something we talked about briefly internally. The current syntax for specifying lookups looks something like this:

```yaml
variables:
VpcId: ${output vpc::VpcId}
```

It allows you to nest outputs, but with some caveats.

It could be nice to support a more sophisticated templating language. Specifically, it would be nice to be able to perform simple transformations on config/outputs before providing them as a variable (e.g. conditionals, math, etc). Not unlike terraform's interpolation: https://www.terraform.io/docs/configuration/interpolation.html#conditionals.