Implement save to disk for h264 video stream

[nassah221]

Addresses #10

For now, the save to disk is enabled at the control level by save_video flag.

I didn't feel that outputs was the right place for configuring the file-writer due to absence of a Listen or Serve method on the writer. Since save-to-disk will be handled on per-stream basis in the future anyways, I thought it better to handle it at the control level.

I also moved keyframer to it's own package and broke up the ingestor, thumbnailer and file-writer code into separate files

[clone1018]

Looks good to me! Uses the channel work you did with Thumbnailer well, and is setup for future codecs.

[clone1018]

You should be able to merge this branch on your own, feel free when you are ready!