Don't patch things unless --ipdb is passed

Currently, the plug-in is still patching `pytest.set_trace` and `ipdb.set_trace` even when `--ipdb` is not passed. This makes it not do that.

Alternatively, we could consider having it patch stuff by default for convenience and then having the user pass `--no-ipdb` if they want to deactivate and keep everything clean and unmodified.