Add callback script for new plugin system in FPP.

• Callbacks script only supports "media" callback, at least for now...

• Use a symlink for main filename and use a ".sh" extension just
  because.

• Update the plugin UI page with information on the new system.
  Eventually FPP will pass us JSON data that contains artist and title
  information parsed from the metadata in the file, rather than having
  simply a filename.