[Request] Thumbnail during inactivity

[boonhapus]

If the feed is unreachable, could we have a thumnail? I'm not confident enough in my async to write a PR, but this would be a great addition that could be pretty simple to do, given your current procedure.

  - platform: ffmpeg
    name: Living Room
    input: -i rtmp://127.0.0.1/live/livingroom -q:v 5
    thumbnail: /path/to/thumbnail.jpg

[pvizeli]

Yeah I can do that