Add ldoc support for generating lua documentation

This is a proposal to add ldoc integration and necessary doc comments for VLCs Lua functions, which will allow to simply run make luadoc to generate lua documentation instead of just having poorly formatted and incomplete README files scattered in the source tree for it.

Of course this is far from complete, I am just publishing the initial draft to get feedback on it and not waste too much time on it in case there is a consensus that for some reason we do not want this approach.

I've additionally explored using Doxygen but it seems nearly impossible to properly generate any form of structured documentation for Lua with it as it was really written with C and C++ in mind.