add eltype for AbstractConfig

[jrevels]

This allows users to easily extract the types of the Dual instances that will be injected into their code:

julia> cfg = ForwardDiff.GradientConfig(prod, rand(20));

julia> eltype(cfg)
ForwardDiff.Dual{ForwardDiff.Tag{Base.#prod,0xb046287d533b082d},Float64,10}

cc @tkoolen

[tkoolen]

Thanks!