avfomat/hlsenc: support fmp4 format in hls

add the fmp4 format into hlsenc
because the fmp4 format add into hls from version 7.
the spec link is:
https://tools.ietf.org/html/draft-pantos-http-live-streaming-20
and the describe on WWDC
https://developer.apple.com/videos/play/wwdc2017/515/



Signed-off-by: Steven Liu <lq@onvideo.cn>