Pretty Printing

It would be good to have a flexible pretty-printer for terms which handled printing to a variety of screen widths, and supported condensed vs. expanded output.

We should implement a clone of https://github.com/quchen/prettyprinter, which is a modern implementation of https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf

We should then have outputs for HTML, console (i.e., colored), and plain text.