Creating duotones from raster imagery

[tristen]

From this style panel:

I'd love to be able to assign a color to the whites and a color to the blacks to create a duotone effect like this:

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I tried a combination of dropping saturation, adjusting brightness, and spinning the hue based on the current raster filtering operations but I'm unable to achieve that exact effect. cc @kkaefer

[lucaswoj]

Merging with mapbox/mapbox-gl-js#3889