ytdl: force "best" format by default

If the --ytdl-format option is not used, force the "best" format. Do
this because youtube-dl is going to change its default format to one
that will trigger the (partially broken) DASH support in our own code.

Fixes #1867.

options/options.c

@@ -691,7 +691,7 @@ const struct MPOpts mp_default_opts = {
 #if HAVE_LUA
     .lua_load_osc = 1,
     .lua_load_ytdl = 1,
-    .lua_ytdl_format = NULL,
+    .lua_ytdl_format = "best",
     .lua_ytdl_raw_options = NULL,
 #endif
     .auto_load_scripts = 1,