Feature Request: optionally only redirect to Catch::cout/cerr if test failed

In my opinion it would be a useful option to redirect both `std::cerr` and `std::cout` into a temporary buffer, which is only printed to console after a test failed.
I was not able to discern if this could be done by writing a new `Reporter`.

In a code I am developing I have a lot of Log messages to help with debugging etc., which I usually do not want to show up for tests which passed. On failing tests, however, the output would often be very useful.