WIP padLinear

[curran]

Closes #150

Please let me know if this looks like a good direction. With some encouragement I'm happy to flesh out the PR with README updates - but first I want to confirm this direction is acceptable. Thanks for reviewing!

[Fil]

It would be nice to see how this would work for other types of scales (log, sqrt…)

[curran]

An idea from https://observablehq.com/@mbostock/inequality-in-american-cities :

function padLog(x, k) {
  return padLinear(x.map(Math.log), k).map(Math.exp);
}