Adds the DifficultyAPI

[harbingerofme]

Used for adding difficulties. This design can probably be generalized to all rule sections.
Because the Difficulty array is in static class as a static readonly array, modifying it is hard. Future patches might change this to a nonreadonly array, as said by Ghor. I'll keep an eye out.
Use:

DifficultyDef myDifficultyDef = new DifficultyDef(myScalingFactor /*(1f=drizzle,2f=rainstorm,3f=monsoon)*/, myNameToken, myIconPath, myDescriptionToken, hoverOverColor);
DifficultyIndex myDifficultyIndex = R2API.DifficultyAPI.AddDifficulty(myDifficultyDef);//pretend this is actual declared inside the valid scope
// Then at any point later you can 
if(Run.Instance && Run.Instance.selectedDifficulty == myDifficultyIndex)
{
//cool code that only happens in your difficulty here.
}

[tristanmcpherson]

Nice! Looks good to me :)