TOOLS/lua/yt-thumbnailer: init

this adds a thumbnailer script that uses pre-generated
youtube-dl thumbnails.

not a fully featured thumbnailer, but useful for testing and as
an example. released into public domain so that 3rd party script
writers can use/copy it for their (hopefully fully featured)
script.

can also be used in case user only wants web thumbnail (since
local files can just be seeked fast).

Author: N-R-K

DOCS/man/osc.rst

@@ -756,3 +756,7 @@ script via the following ``user-data`` properties:
 
     The OSC sets this property to ``nil`` to signal the thumbnailer to clear
     the displayed thumbnail.
+
+The script ``TOOLS/lua/yt-thumbnailer.lua`` implements this API to fetch and
+show pre-generated storyboard images available through the youtube-dl hook
+script on certain sites (e.g youtube).