embed c3.js & d3.js into the generated HTML

to make it work standalone, without relying on CDNs being available. As a build step, we could

- add `{{c3css}}`, `{{c3js}}` & `{{d3js}}` placeholders to `lib/head.html`,
- fetch the JS/CSS code from [`cdnjs.com`](https://cdnjs.com),
- replace the placeholders

Start by reading lib/head.html and the build step that generates the HTML. Add the c3css, c3js, and d3js placeholders, fetch the JavaScript and CSS from cdnjs, and replace the placeholders so the generated chart works standalone without relying on a CDN.

Written by the indexing model from the issue text.