Add get parameter to change behavior

GET PARAMETERS ACCEPTED :
    - mode :
       * 'cover' : look for a 'Folder.jpg' file in the folder of the
song passed to the script
       * 'song' : look for an artwork in the metadata of the song
passed to the script
       * 'folder' : look for an artwork in the metadata of all the
songs stored in the folder of the song passed to the script
       * novalue : try 'cover', if fail then try 'song', if fail then
try 'folder'
    - cache :
       * 'off' : don't store and restore in APC the artworks of the
last songs passed to the script
       * novalue : store and restore in APC the artworks of the last
songs passed to the script

  ie :
http://volumio.local/artwork/music/NAS%2FMusic%20Tib%2F2080%2FThe%20Back
up%20-%20EP%2FFolder.jpg?mode=folder&cache=off