add CLI command to show workflow termination info

[cmarcelk]

My understanding is that the server has an API for getting the metadata that is stored upon workflow termination, both in the case of normal and abnormal termination. Would be very nice to get access to this for debugging workflows. For example "relay workflow:logs:list --name mywf" to list out the last X instances of the named workflow, and "relay workflow:logs:get --name mywf --instance myinstanceid" to show the stored metadata of a specific instance.