Refactor to add some useful CLI options

[cemulate]

Hello! I wanted to modify this program for some of my use cases, and decided it would be easy enough to implement these cases as CLI options. As such, this is a sizeable PR with quite a bit of refactoring; I'm completely open to different naming, etc., if you have opinions.

I've added CLI options:

options:
  -h, --help            show this help message and exit
  -r, --preserve-roman  Do not italicize roman letters (A-Z a-z) in math
  -w, --preserve-math-whitespace
                        Preserve whitespace between math characters
  -p PREAMBLE, --preamble PREAMBLE
                        Override data file from a JSON file with custom commands

The first two are self-explanatory, and the preamble option is an extensible solution for users that wish to change or alter symbols from unicodemathsymbols.txt (or use their own "macros") without relying on the program author to make such a change.

All the existing tests pass, and I've added a few tests for the CLI options as well.