Add an option for AttachMe to be quiet on stdout/stderr

I'm debugging JVM processes that get called by other programs and whose stdout/stderr get examined. AttachMe messages to stdout/stderr break the calling process by introducing unexpected output.

It would be nice to add an additional agent option (maybe `quiet=[y/n]`) to avoid printing anything, at least for anything that is not a fatal error. That would make it possible to debug JVM cli apps that get called from scripts, etc.