Include an API for SVG transform attributes

d3-transform makes it easy to define and reuse functions that produce transform attribute strings for SVG elements. Using d3-transform reduces repetition, allows you to compose multiple transforms, and eliminates ugly string-interpolation from transform attr calls.

More info and docs here: https://github.com/trinary/d3-transform#readme

This PR includes that transform API and adapts the code and tests in that repo to fit d3's existing structure and style.