Custom interface with Pandoc

[kmaasrud]

Pandoc currently works as a Markdown parser (for converting to LaTeX) and citation processor. I want this functionality to be pluggable, in case I want to remove the Pandoc dependency in the future.

Because of this, I need a Pandoc API. The pandoc crate exists, but it is not very elegantly written, and even limiting in some cases. Thus, I will have to write my own API, but it can be simple and specialized.