Excluding recorded_at

[aiguofer]

Since cassettes tend to get checked in to VC, it would make sense to have an option to remove the recorded_at key from each interaction. This would make the diffs smaller when tests are re-recorded.

Looking at the code, the only reference I can see for this property is in Cassette.earliest_recorded_date, which seems to be used to determine whether a cassette needs to be re-recorded. Would it make sense to just add a timestamp indicating the beginning of the recording to the cassette itself?