Add settings to support poll-only watchdog for autoreload

- SUPERVISOR_AUTORELOAD_POLL_ONLY: Set to True to us only PollingObserver.

- SUPERVISOR_AUTORELOAD_TIMEOUT: Set to higher number to reduce polling observer's CPU usage, at expense of higher autoreload latency.
Merge pull request #1 from fanscribed/master

Add settings to support poll-only watchdog for autoreload