Support using custom color names

[sehilyi]

Fix #60 (review)

import { GetColorName } from 'color-hex-to-color-name';
// ...
<AltGoslingComponent
  spec={spec}
  // user can define their own color names
  customColorNames={(hex: string) => GetColorName(hex)}
/>

@thomcsmits Do you think this is something that we can add to the AltGosling?

[sehilyi]

I think this is a lower priority since we figured out the issue of color names.

[thomcsmits]

Yes, but still great support to have! I'll take a look soon!