Fix unary operator expected

Added in a way to easily inspect just the output of the command under the scope to more easily catch things like this

```
cucumber features/git-prune-branches.feature OUTPUT_COMMANDS=1
```

Tacked on a couple more commits fixing bugs founds using OUTPUT_COMMANDS
* Git extract had some similar errors
* Abort / continue checks weren't working properly (git extract was listing a continue message and trying to remove a non existent continue file)
Shouldn't `debug` always default to `ENV['OUTPUT_COMMANDS']`? Or since you check for `ENV["DEBUG"]` inside of `run`, maybe check there for `ENV['OUTPUT_COMMANDS']` as well? It seems strange and unintuitive that both methods take the same parameter, one calls the other, but they have slightly different default values.