text: Title: Allow using `pytest-recording` with a command-line flag only
  Body: It seems like right now you need to define which functions will make use of `vcr` with a decorator. It would be great if there was a way to automatically enable `vcr` for all tests without requiring any change in the source code.
  
  For example, having a flag like `pytest --vcr` would automatically enable VCR for all requests.
  
  This way, tests would run in any environment without `vcr` nor `pytest-recording` installed (i.e.: CI/CD/deployment) and, for those wanting to accelerate local testing, it would be as simple as installing the packages and adding the `--vcr` flag.
  
  Not sure this is even possible. :shrug: :smile: 