Multi print page

[bobneuman]

This adds the ability to define multiple pdfs by creating a dictionary of print_page settings with the key set to the print page name to generate. It uses the original configuration inputs as globals for print_pages unless the print page config overrides the setting. If print_pages is not defined within the mkdocs.yml then the configuration inputs add a single print_page item in the plugin print_pages.

This also resolves #76 by inspecting for the presence of child items within a section before adding to the TOC and Menu.

[timvink]

I (finally) went through this PR. It indeed looks like working way to define multiple print pages for a given website.

I am however concerned that this is adding a lot of additional logic (and an extra option) for what is a niche use-case that very few users will have.

I also think the same can be achieved by splitting up your site into multiple smaller sites (that can each have a print page) using the projects plugin. In that case we could just write an instruction in the docs (docs/howto) on how to set this up