Add channelcount information on HLS audio tracks

[avelad]

According to https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.1

CHANNELS

The value is a quoted-string that specifies an ordered, "/"-separated list of parameters. If the TYPE attribute is AUDIO then the first parameter is a count of audio channels expressed as a decimal-integer, indicating the maximum number of independent, simultaneous audio channels present in any Media Segment in the Rendition. For example, an AC-3 5.1 rendition would have a CHANNELS="6" attribute. No other CHANNELS parameters are currently defined.

All audio EXT-X-MEDIA tags SHOULD have a CHANNELS attribute. If a Master Playlist contains two renditions encoded with the same codec but a different number of channels, then the CHANNELS attribute is REQUIRED; otherwise it is OPTIONAL.

This issue is related to: #424

Sample:
https://tungsten.aaplimg.com/VOD/bipbop_adv_fmp4_example/master.m3u8
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="aud1",LANGUAGE="en",NAME="English",AUTOSELECT=YES,DEFAULT=YES,CHANNELS="2",URI="a1/prog_index.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="aud2",LANGUAGE="en",NAME="English",AUTOSELECT=YES,DEFAULT=YES,CHANNELS="6",URI="a2/prog_index.m3u8"
#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="aud3",LANGUAGE="en",NAME="English",AUTOSELECT=YES,DEFAULT=YES,CHANNELS="6",URI="a3/prog_index.m3u8"

[ismena]

@avelad thanks for bringing this up, I think we could definitely add this for hls!
Let me schedule it for 2.2 unless @joeyparrish has objections.

[joeyparrish]

Sounds good to me. And the spec is more clear than the corresponding feature in DASH. :-)