Use nograd keyword for functions too

[mcabbott]

julia> using Tullio, Tracker, ForwardDiff

julia> plus10(x) = x + 10; # a function unknown to DiffRules

julia> Tracker.gradient(rand(3)) do x
         @tullio res := x[k] * plus10(k)  verbose=true
       end
┌ Warning: symbolic gradient failed
│   err = "no diffrule found for function plus10(_)."
└ @ Tullio ~/.julia/dev/Tullio/src/macro.jl:1260
ERROR: no gradient definition here!

julia> Tracker.gradient(rand(3)) do x
         @tullio res := x[k] * plus10(k)  grad=Dual  # old solution
       end
([11.0, 12.0, 13.0] (tracked),)

julia> Tracker.gradient(rand(3)) do x
         @tullio res := x[k] * plus10(k)  nograd=plus10  # new solution
       end
([11.0, 12.0, 13.0] (tracked),)

Needs a test, but #57 is re-organising them.