external_downloader_args: A list of additional command-line arguments for the
external downloader.
hls_use_mpegts: Use the mpegts container for HLS videos.
+ http_chunk_size: Size of a chunk for chunk-based HTTP downloading. May be
+ useful for bypassing bandwidth throttling imposed by
+ a webserver (experimental)
Subclasses of this one must re-define the real_download method.
"""