coin2html: use webpack to produce the html page

[mkobetic]

The simplicity of emitting the html page directly was nice, but the main drawback is that because the typescript code was not modularized it was hard to test. I tried to find a way to avoid a builder, but even with the ES module support in the browser there's no way to import a module inlined within the page, see e.g. https://stackoverflow.com/questions/43817297/inlining-ecmascript-modules-in-html. I read somewhere that it is forbidden by the spec (don't have a reference anymore). There are ways to hack something in, but that defeats the simplicity goal.

So this PR adds webpack configured to spit out a single HTML page with everything built into it, except the imports like d3.js. This allows splitting the monolithic source file into models and views modules. It adds jest config and a first model test, more will follow over time.

Still trying to keep it minimal, just npm, webpack, linters and jest. See package.json for available scripts.