functionality for the derivative of atan2

I am a JuMP user that wanted to be able to use the `atan2` function and I noticed that it was on someones radar [here](https://github.com/johnmyleswhite/Calculus.jl/blob/cb42f3699177449a42bdc3461c8aea8777aa8c39/src/differentiate.jl#L254) to add this functionality and I am wondering is this straightforward to do or would it be difficult to do and extend to JuMP since `atan2` is a function of two variables. @mlubin what do you think?