reqtest: Add RecorderMode

[earthboundkid]

When writing tests you want to record responses, but then once you get done writing tests, you want it to be an error to issue a request without a corresponding entry. You can do this by just switching from reqtest.Caching to reqtest.Replay, but it would be more convenient if there were a single function that just took one of three mode arguments.

[acehinnnqru]

Do you have any code sample?

[earthboundkid]

See #124.