text: Title: CLI option to update snapshots without running other tests
  Body: **Is your feature request related to a problem? Please describe.**
  
  I am using syrupy in a few libraries, and I find myself in a common pattern in which I write a Makefile target like `make snapshot-update` which invokes `pytest --snapshot-update`.  But the real intention behind that make target is not necessarily to run all the tests _and_ update snapshots; but to update the snapshots only.
  
  **Describe the solution you'd like**
  
  I think it'd be a neat feature to support a pytest selector which isolates only the tests depending on the snapshot fixture; so that `pytest --snapshot-update` could be extended to only run tests with any snapshots.
  
  **Describe alternatives you've considered**
  
  It's not a big deal! I just run all my tests when i run updates. :smile: 
  
  **Additional context**
  
  <!-- Add any other context or screenshots about the feature request here. -->
  