Allow controlling the JND of gamut mapping

[facelessuser]

Current gamut mapping has a set JND, but certain things, like tonal palettes, need to be able to gamut map tighter. The plugins should allow configuring this, and fit() should probably be able to handle additional options for a gamut map method.

[facelessuser]

This is really important for every day gamut mapping and specialized that may require more fine tuning. Tighter requirements may cause more iterations, and may not be desired as a default, but for some things, it may be desired, like tonal maps.