Added a bibliography system using citation-js

[Drew-S]

With the added bibliography system users only need to use +cite(key) to create an (in text) citation. Then add a +bibliography(style) to the bottom. The system uses citation-js so keys and styles need to be supported values. Style should be 'citation-apa'. Currently I have not figured out a way to use a local bibtex file library yet.

[Zulko]

This is cool, but my main objections are (1) it adds a Javascript import in the page by default, while it should only import it when bibliography is used, and (2) it runs javascript at page rendering time, which I want to avoid if possible, the page should be as "definitive" as possible when fed to chromium. The reason is that right now there is no way to know that the javascript has totally ended before rendering the page, so for big documents the bibliography may be skipped without a warning. There may be a way to run the bibliography at pug compilation time some way.

If you solve (1) I can merge it, for (2) I'll try a bit when I have time.