Enable Printing and PDF Verison

[stefanpie]

Would it be possible to enable printing and a PDF version of the book? This would be a helpful accessibility feature and would allow users to reference the material offline. I’m not sure why HTML printing is explicitly disabled.

Below are the small changes to book.toml that would be needed to enable these features:

[output.html.print]
enable = true
page-break = true

[output.pdf]
enable = true

Note: PDF support would also require the mdbook-pdf crate.

[tomtau]

printing was disabled in this PR #35 by @AntoniosBarotsis but I don't remember the exact context whether it was causing some issues