Add option to clear tags on VLC close with time to kill

[ophirhan]

Add an option in the main dialog to clear the tags of the current files on close.
If the user decide to clear the tags on close, set tags.time_to_kill = now().
if the user decides not to clear the tags on close, set tags.time_to_kill = a month from now (arbitrary).
On vlc close:

for video, tags pairs(config.CNSR) do
  if now() > tags.time_to_kill then
    tags = nil
  end
end