Support writing HLS using PyavOutput

[davidplowman]

There were just a couple of missing things:

1. We need to be able to pass in options such as hls_time etc.
2. Packets must correctly flag when they are keyframes.

You can make a PyavOutput write HLS using, for example:

options = {
"hls_time": "5",
"hls_list_size": "8",
"hls_flags": "delete_segments+append_list+independent_segments"
}
output = PyavOutput("/dev/shm/cam_hls/live.m3u8", format='hls', options=options)