start/stop object tracking from command line

/to @rmg or @kraman 

I need to update README, and read my own code over again, and do some manual testing on usability of cli messages, but it appears to  work.

Also, in tests, I'm not verifying that the metrics are sent... should be easy by listening on the UDP metrics port, but that's for tomorrow.

But basically, its here, ready for pre-review.
Should we document this as `sl-runctl` instead of `slrc`? The former is the official name, right?