Add command logging support

[lowell80]

All CLI calls should be (optionally) logged to a central log file. For the Splunk app install, this should be easy enough ($SPLUNK_HOME/var/log/splunk/ksconf-exec.log, or something like that.) But for standard installs this needs to be thought through more.

The log format should be single-line JSON objects.

Needs props.conf entry and possibly a simple search / dashboard.

Need to make sure we can easily globally disable this for things like unittesting.

Long term this could be helpful for capturing what commands and arguments are most frequently used. Possibly with an easy way for users to anonymize and submit some aggregate usage data (opt-in only, of course).