Generate PDFs using Puppeteer and Paged.js

[nokome]

Given that we integrate Puppeteer, there is an opportunity to generate PDFs using that, rather than requiring users to have a Latex tool like TeXLive installed. This could be combined with Paged.js to create beautiful paginated PDFs of documents.

Relevant links:

• https://www.pagedmedia.org/a-year-in-the-paginated-world/
• https://gist.github.com/glenhallworthreadify/d447e9d6b1fc9cb807b46f952236d4bc
• https://medium.com/@raphaelstaebler/advanced-pdf-generation-for-node-js-using-puppeteer-e168253e159c

[nokome]

Closing this as we now have PDF generation and paged media will be introduced into PDFs and HTML via themes from https://github.com/stencila/thema