Add --hls-use-mpegts option

When using the mpegts container hls vidoes can be played while being downloaded (useful if you are recording a live stream).
VLC and mpv play them file, but QuickTime doesn't.

youtube_dl/downloader/common.py

@@ -45,6 +45,7 @@ class FileDownloader(object):
                         (experimental)
     external_downloader_args:  A list of additional command-line arguments for the
                         external downloader.
+    hls_use_mpegts:     Use the mpegts container for HLS videos.
 
     Subclasses of this one must re-define the real_download method.
     """