Add output access callback #270

[agentgt]

This callback in the output registry allows one to safely perform operations on the LogOutput by using the appender lock mechanism to help continue keeping locking/reentry protection out of LogOutput.

Currently it only performs one operation on the named appender. Perhaps in the future we allow a callback that is void and is given more meta data about the appender but for now this will help meet the need of someone buffering a window LogOutput data to be fetched. See #269 .