`show` that only shows non-default values

Would be nice to omit default values in show() of structs that have defaults:
```julia
julia> @kwdef struct S
       a = 1
       b = 2
       end

julia> @batteries S kwshow=true

julia> S()
S(a = 1, b = 2)  # could be S()

julia> S(b=10)
S(a = 1, b = 10)  # could be S(b = 10)
```
Especially useful for large structs, but nice even with only a couple of parameters.