Add .js source code beautifier and config file

Neither js-beautify nor code-painter can handle the (bonkers!) way @nickdesaulniers likes his function definitions, but esformatter can. It's a bit alpha and pretty slow, but it does a good job. I haven't checked in the results of running it yet as there are PRs outstanding, but locally, they look good.

Run it for yourself with `npm run-script beaut`

The config in .esformatter is based on this: https://github.com/millermedeiros/esformatter/blob/master/lib/preset/default.json

Please tweak to your liking, then perhaps out a note in the readme?