d3.interpolateTransform that takes a matrix.

[mbostock]

I’d like a version of d3.interpolateTransform that, instead of taking a transform string as d3.interpolateTransformCss and d3.interpolateTransformSvg do, takes an array of six numbers [a, b, c, d, t_x, t_y] in the same manner as the matrix transform function.

This would potentially eliminate the need for separate CSS- and SVG-specific transform methods, in conjunction with changing how the transform style and attribute is handling in d3-transition (d3/d3-transition#72).