feature request: workflow log command

[deanmalmgren]

It might be convenient to store more than just the last workflow's output so you can see historical information about when a workflow was run. Maybe we could start to store ~10MB of historical logs in .workflow/log/YYYYMMDDHHMMSS.log files that correspond with the timestamp that the workflow was first started. Then we could have a workflow log command that would display the logs in reverse chronological order.

This came up in creating the functional tests for --skip which directly reads a hidden file (doesn't seem like a good idea) but I could imagine this being useful in other contexts as well.