Option to allow original tracks for parts of the level by disabling MainMusic/BossMusic for a mod

[theamazingfedex]

Under the options for an installed mod, provide the ability to disable either the MainMusic or BossMusic sections of the mod (but not both) to allow the game to fall back to the original track for that level. Useful for when a boss track is missing on a downloaded mod, or you just don't want to hear the same song over again.