Added Volume and Pitch as options for Mod:onMapMusic()

[DiamondBor]

now if you'd like to play your map's music at double the volume and half the pitch, you'd do

function Mod:onMapMusic(map, music)
    return {music, 2, 0.5}
end

and, it has compatibility with the old version of the transitionMusic() function, so you can still use it the old way:

function Mod:onMapMusic(map, music)
    return music
end