Write a processor CLI

[bdarcus]

Along with #2, the main entry-point should be extended to mirror the haskell citeproc one, but make sure it also aligns with #5.

https://www.makeuseof.com/nodejs-cli-packages-build-tools-best/

Deno-specific:

https://cliffy.io/

Or perhaps this should just be a library, and the CLI a separate project?

Regardless of those details, need a CLI.