Linear RGB type

I would appreciate it if Colors.jl provided a linear RGB type which acts as follows. Thank you!

```jl
    @test convert(RGB, LinearRGB(0.5, 0, 0)) ≈ RGB(0.7353569830524494, 0, 0) atol=eps()
    @test convert(LinearRGB, RGB(0.7353569830524494, 0, 0)) ≈ LinearRGB(0.5, 0, 0) atol=eps()
```
