Add the ability to disable recording

[tahirmt]

In assertSnapshot it would be nice to have an option to opt out of recording altogether even with record mode turned on or missing snapshot.

Use Case

Having a test that expects a pre-existing snapshot as a result of a previous assertion and then does something to test. This is useful if setting up a test cannot be validated except through the use of a snapshot.

[stephencelis]

There is an open PR tracking this here: #531

We haven't committed to the feature yet, but we're tracking it, thanks!