Non-parametric colors fail in addition and subtraction

[kimikage]

This problem is noted in #56, but the addition and subtraction have not been fixed.

julia> RGB{N0f8}(1, 0, 0) + RGB{N0f8}(0, 0, 1)
RGB{N0f8}(1.0,0.0,1.0)

julia> RGB24(1, 0, 0) + RGB24(0, 0, 1)
ERROR: TypeError: in Type{...} expression, expected UnionAll, got Type{RGB24}

julia> Gray{N0f8}(1) - Gray{N0f8}(1)
Gray{N0f8}(0.0)

julia> Gray24(1) - Gray24(1)
ERROR: TypeError: in Type{...} expression, expected UnionAll, got Type{Gray24}