Initial implementation of checkbox component

[lmorchard]

• Also small tweak to ensure stylelint catches everything on Windows

Fixes #20

[lmorchard]

Rebased against master, mostly to see if the new storybook-per-PR stuff would run

[lmorchard]

Hmm, so the script for per-PR storybook builds didn't manage to post a comment here. But, it did deploy storybook for this checkbox PR:

https://FirefoxUX.github.io/photon-components-web/branches/20-checkbox

[lmorchard]

Ugh, going to try re-opening this from a branch on this repo, rather than from my fork.