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)
Since the parameters of these two methods completely match, you could just pass on the parameters here, and determine the default values in `run`s declaration.

```ruby
def run_and_store *args
  @last_run_result = run *args
end
```