Allow using on lab::Lab about kmeans-colors

[okaneco]

I'll try to add that as a feature soon.

In the meantime to get up and running, you could wrap it in a newtype then copy the current Lab implementation. But that's obviously cumbersome and not ideal.

As for LCh, I didn't implement it originally because the results end up being the same as Lab, it's just a different representation of the same space. It ends up slower because you now have to do 2 extra conversions of Lab->LCh->Lab. You can just convert the resulting Lab centroids to LCh after the fact.