Merge with ARS.jl #25

Hi! I and thanks for this package! I have made a different implementation (ARS.jl) which adds some features which I found necessary as well as significantly faster sampling (from some limited testing. The features I found missing were:

1. Allowing different numeric types than Float64 in the sampler (I needed to use BigFloat).
2. Allow the user to choose autodiff backend through DifferentiationInterface.jl (not all autodiff libraries are compatible with all numeric types).

Instead of having 2 slightly different libraries registered I would prefer merging mine with yours. However, this would probably result in a breaking release and replace most of the internals. Would a PR accomplishing this be welcome?

ARS.jl repo: https://github.com/Eliassj/ARS.jl