container_output() method for streaming the output without logs=1

I need to stream a container's output, *with* the parsing/processing of frames, but *without* the backlog. The `logs` method seems to be the only one that does that processing, so I've extracted the body of `logs` and added a parameter. `logs` just calls `container_output` with `logs=True`.

The semantics of `logs` vs `attach` is perhaps getting a bit confusing, and there's some repetition - I wonder if it can all be simplified somehow.