Added ability to invert relative mouse movement to analog stick

[MungoRae]

I normally play with my Y axis inverted and I wanted to be able to specify mouse movement like

# Map relative mouse movement to a analog stick
REL_X = right_analog_x
REL_Y = -right_analog_y

So I have added in some code that allows me to do this.

[Ape]

Thanks! I reviewed the code and tested that this works.