Upstreaming various changes from fork

[connorjclark]

Hi Evan!

The Lighthouse team forked this repo: https://github.com/paulirish/webtreemap-cdt

We will be using webtreemap for a new feature in Lighthouse, @paulirish is also working on a new feature in Chrome DevTools that uses it too.

We made a few changes in that fork:

a) webpack -> rollup (not necessary to upstream imo)
b) add option for not injecting CSS with JS
c) split out a public layout() method
d) a11y: keyboard navigation
e) options.spacing

Would you be open to receiving some PRs for upstreaming these changes?

[evmar]

Thanks for reaching out, and sorry I am so slow to respond! I filed #37 about the future of this project, and I'd love your feedback there.