lua: playlist: add a way to change playlist meta

Expose a way to change playlist metadata in Lua. For now add a specific call to the vlc interface only available in the parse function, it should be used that way:

function parse()
-- This is the usual "meta" Table also used on returned items.
vlc.set_current_playlist_meta({
title="Podcast title",
arturl="https://artworkuri.jpg",
})

-- ...
end