text: Title: Add option to add non-existing snapshots instead of raising an error
  Body: **Is your feature request related to a problem? Please describe.**
  
  The issue is that I have to run `--snapshot-update` in order to initiate a new assertion in a test. This is mostly an annoyance when testing with PyCharm using the IDE testing tools.
  
  **Describe the solution you'd like**
  
  I would prefer that there is an option that I can set by default that will not update any existing snapshots, but will add any newly created snapshots. 
  
  **Describe alternatives you've considered**
  
  A flag called `create-non-existing-snapshots` as an option that can be set. This could then be configured in CLI or an IDE.
  
  **Additional context**
  
  None