Request: Add separate motion start/stop alerts

[luma]

Sighthound has a unique value proposition for home automation. Existing motion sensors are not very good at discriminating between people and pets, something with SH does a pretty good job of. The ability to execute commands in response to motion is super helpful, as this allows me to reach into my HA platform to trigger events based on Sighthound's pedestrian tracking algorithms (something I never really could get dialed in with OpenCV - you guys are wizards). What would make it even better would be the simple addition of separate commands for motion start and motion stop. This would let HA users handle motion events in a far more powerful way and would make SH even more valuable for this use case.