As of 079ecd7 (2014-11-19), the hook script is now integrated into mpv and will be avaiblable when compiled with LUA support. It is (currently) disabled by default, to enable it add --ytdl=yes to your command-line or ytdl=yes to your mpv.conf.
- automatic selection of ASS subformat (rg3/youtube-dl#4019)
- youtube-dl may get stuck on direct URLs to less popular file types (m2ts) (rg3/youtube-dl#4149)