Debugging commands

[christophermaier]

Provide per invocation / pipeline log viewing (e.g., what data came into this command, what went out of it).

Once we can send multiple outputs from a pipeline, this would be easy to do with an inspect command. Think something like my-bundle:do-it --foo=bar | inspect | other-bundle:do-more | inspect, where each inspect call sends a message showing the output of the previous command, while also piping it through to the following.

Another idea involves providing a special "debug" message queue topic to which commands can post information. This would probably be presented back to the user in a way similar to the above inspect command.