Have the ability to add a bibliography to a posting

[cdchapman]

See https://pandoc.org/MANUAL.html#citations for Pandoc's citation processing documentation.

A few things we should support

• Styles should be settable using Citation Style Language
• Use an external bibliography file in the item's directory
• Be able to pull in bibliography metadata from other locations, specified in nanoc.yml
• Use YAML metadata as another option

Check out Ruben's nanoc plugin for ideas: https://github.com/RubenVerborgh/nanoc-bibtex