Allow users to inject custom interpolators

[mcnuttandrew]

This PR addresses the concerns raised in #42 by following @mbostock's suggestions in the same thread. Specifically: it presents a new function, customInterpolate, that allows users to specify an object of functions that will override the default versions of those interpolators.

This can be useful if you want use the top level interpolator, but don't want to interpolate through RGB, or if you want to prune children from a tree you are interpolating against, or simply just don't want strings to be interpolated.

[mcnuttandrew]

@mbostock bump for review?