Feature Request: Automatically deleting unused snapshots

[ngc]

We want to add pytest-insta to our codebase for snapshot testing but unfortunately it doesn't seem to delete no longer used snapshots?

Reproduction steps

1. Create a test with a snapshot
2. Run tests
3. Delete the test
4. Run tests again
   Result: The snapshot will still be there

I'll see if I can put up a PR to fix this.

[cjw296]

@ngc - did you end up getting a PR together for this?