Test: show/write generated source code

I don't expect this PR to merged. It's just a proposal for a feature - I found this useful for myself...

Basically my dev workflow is, running feature tests all the time (most of the time just a single spec). I wanna see the generated output to make sure it looks the way I want it... This outputs the generated code into a separate file, which I keep open in my editor and it keeps updating...

```bash
# output transpiled code into a file
OUTPUT=file make test/features

# output transpiled code into stdout
OUTPUT=console make test/features
```