nalgebra support in pixelcolor

[tommy-gilligan]

Being able to transform colors to/from nalgebra vectors can be pretty handy for certain applications.
Seeing as though nalgebra is already an optional dependency, it is perhaps a sensible enhancement. Or it is scope creep and belongs outside of this crate? What do you reckon?

[rfuest]

Do you have a specific use case in mind? It's a bit of feature creep, but as you said we already have the optional nalgebra dependency anyway and I would be OK with adding this.