(Better) export to PDF

This is a continuation of https://github.com/hackmdio/codimd/issues/33.

There seem to be a number of issues requesting a better way to export notes to pdf. To the best of my understanding, currently exporting notes to pdf is only allowed in slide mode, and via an error-prone and not user-friendly procedure that involves modifying the URL manually. There has to be a better way.

Exporting to pdf is invaluable for a number of reasons: it's usually easier to read pdfs on mobile devices than web pages, it's much easier to print and can be accessed offline (e.g., on a plane). Can you please work on a way to do so that works in all modes (view, slide, book) and is user-friendly? Thank you!