shorthand for scale type and color scheme

[mbostock]

I want to be able to say

color: "blues"

as shorthand for

color: {scheme: "blues"}

and likewise I could imagine

y: "sqrt"

as shorthand for

y: {type: "sqrt"}

Basically for any scale option that’s defined as a string, we check if it’s a valid scale type. And for color, we also check if it’s a valid color scheme. (These sets should be non-overlapping.) We do something similar for the projection option in #1111 and it’s nice.