Add a check command

[Kriskras99]

The check command should check the following:

• Is the song missing any textures/classifiers
• Does the song have any 'inferior' assets
  • map_bkg sourced from banner_bkg (which has wrong colours)
  • Asset sourced from older game or JDNow, when it's also available in a newer (supported) game
• Which songs are missing (this requires keeping a list of all songs up to date)
• Which songs are censored (see Add an uncensor command #16)