supprt new mode of read write

Hi,
If i understood correctly, flashback support only 2 modes: record & play. This is great but IMO it misses very important mode. In Betamx, it was called READ_WRITE mode.
Lets say i have a scene file with 1 recorder http request-response from web service1. Now, lets say i want to record another request-response from another service2. If i set mode to record, it will overwrite the already recorded service1 request-response.
So what i want to suggest is to add another mode READ_WRITE . In this mode first we read the scene file if there is a match, play the recorded one, if there isn't go outside and record the new one. The new one will be appended to the file and will not override the previous recorder ones. Just like READ_WRITE in betamax.

IMO this feature is must, i'm using it all the time

Review Flashback's existing record and play mode implementation and how scene files are matched and updated. Define the READ_WRITE behavior from the issue: replay matching requests, record unmatched requests, and append them without overwriting existing interactions; verify both paths with the project's tests.

Written by the indexing model from the issue text.