ytdl_hook: Allow loading youtube-dl.py on Windows

I'm not sure about actually using 0 and 1 for the values of pyscript, but I was getting too many warnings/errors when I tried setting it up with other values.  I've never done any Lua scripting before, so it can probably be simplified over what I did.

I'm also not sure if it tries using both when both youtube-dl.exe and youtube-dl.py are present and available for use, or if it only uses one of them.  That scenario should be rare, though.

Searching the %PATH% doesn't work for youtube-dl.py because Python doesn't support that, apparently (unless I'm missing something obvious).  So it has to be in mpv's root directory or symlinked there.