Make it easy for users to inject derivative definitions

[jrevels]

Just realized I misinterpreted the point of #5, so I'm superceding that issue with this one. We should probably have a mechanism for user-defined derivatives that works for n-ary, multivariate functions.

[samuela]

Is there a way to accomplish this at the moment?

[ChrisRackauckas]

It was completed in #123 and has the same semantics as Zygote.@adjoint. I think @oxinabox is going to make it connect to ChainRules.jl soon too.

[oxinabox]

@mohamed82008 is going to do it soon, but is currently thesis stuff,
I am going to finish making Nabla.jl do this soon, so there will be something difficult to base it on.
(There are examples in the ChainRulesCore docs, but once you want to do a real AD handling every edge case it is a bit more complex.)