HLS custom tags

[toots]

This PR adds the ability to define extra tags on HLS playlists. It adds 3 distinct ways of doing it:

• The extra_tags argument on the operator adds extra tags to the main playlist.
• The optional extra_tags method on the stream list adds extra tags to the given stream.
• The return output (of unit) type has a streams method which returns the list of streams with their attributes (encoder, name, bandwidth) and a insert_tag to insert a tag into their playlist
• Tags inserted via the insert_tag cause the current segment to end. The tag is the written to the playlist and a new segment begins.

Example:

Script:

# Add X-CUSTOM-TAG to the main playlist:
o = output.file.hls(
  extra_tags["X-CUSTOM-TAG"],
 "/tmp/hls",
  [(
    "stream", 
    %ffmpeg(format="mpegts", %audio(codec="aac")).{
        # Adds an extra tag to this stream.
       extra_tags = ["X-CUSTOM-STREAM-TAG"]
    })
  ]
  s
)

# Insert a tag to all streams
list.iter(fun s -> s.insert_tag("X-CUSTOM-STREAM-TAG"), o.streams())